MenuItem and toolbar missing in new activity Android Studio Unknown Bug

I am completely new to Android Studio and just learned Object-oriented programming. My project requires me to build something on open-source code. I added a new menu item to a menu and want to start another activity once the user clicks the menu item with id: plot. The intent will be sent from if (id == R.id.plot) within onOptionsItemSelected(MenuItem item) of TerminalFragment. For building activity_main2 with a menu, I followed a recipe on the internet to do it. I found out that the page of activity_main2 is blank (all black, no either toolbar or menu) when testing. What is missing with my code?

TerminalFragment

package de.kai_morich.simple_bluetooth_le_terminal;

import android.app.Activity;
import android.app.AlertDialog;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.text.Editable;
import android.text.Spannable;
import android.text.SpannableStringBuilder;
import android.text.method.ScrollingMovementMethod;
import android.text.style.ForegroundColorSpan;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import android.widget.EditText;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class TerminalFragment extends Fragment implements ServiceConnection, SerialListener {

    private MenuItem menuItem;
    private enum Connected { False, Pending, True }

    private String deviceAddress;
    private SerialService service;

    private TextView receiveText;
    private TextView sendText;
    private TextUtil.HexWatcher hexWatcher;

    private Connected connected = Connected.False;
    private boolean initialStart = true;
    private boolean hexEnabled = false;
    private boolean pendingNewline = false;
    private String newline = TextUtil.newline_crlf;

    /*
     * Lifecycle
     */
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
        //Register with activity
        // You must inform the system that your app bar fragment is participating in the population of the options menu.
        // tells the system that your fragment would like to receive menu-related callbacks.
        setRetainInstance(true);
        deviceAddress = getArguments().getString("device");
    }

    @Override
    public void onDestroy() {
        if (connected != Connected.False)
            disconnect();
        getActivity().stopService(new Intent(getActivity(), SerialService.class));
        super.onDestroy();
    }

    @Override
    public void onStart() {
        super.onStart();
        if(service != null)
            service.attach(this);
        else
            getActivity().startService(new Intent(getActivity(), SerialService.class)); // prevents service destroy on unbind from recreated activity caused by orientation change
    }

    @Override
    public void onStop() {
        if(service != null && !getActivity().isChangingConfigurations())
            service.detach();
        super.onStop();
    }

    @SuppressWarnings("deprecation") // onAttach(context) was added with API 23. onAttach(activity) works for all API versions
    @Override
    public void onAttach(@NonNull Activity activity) {
        super.onAttach(activity);
        getActivity().bindService(new Intent(getActivity(), SerialService.class), this, Context.BIND_AUTO_CREATE);
    }

    @Override
    public void onDetach() {
        try { getActivity().unbindService(this); } catch(Exception ignored) {}
        super.onDetach();
    }

    @Override
    public void onResume() {
        super.onResume();
        if(initialStart && service != null) {
            initialStart = false;
            getActivity().runOnUiThread(this::connect);
        }
    }

    @Override
    public void onServiceConnected(ComponentName name, IBinder binder) {
        service = ((SerialService.SerialBinder) binder).getService();
        service.attach(this);
        if(initialStart && isResumed()) {
            initialStart = false;
            getActivity().runOnUiThread(this::connect);
        }
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {
        service = null;
    }

    /*
     * UI
     */
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_terminal, container, false);
        receiveText = view.findViewById(R.id.receive_text);                          // TextView performance decreases with number of spans
        receiveText.setTextColor(getResources().getColor(R.color.colorRecieveText)); // set as default color to reduce number of spans
        receiveText.setMovementMethod(ScrollingMovementMethod.getInstance());

        sendText = view.findViewById(R.id.send_text);
        hexWatcher = new TextUtil.HexWatcher(sendText);
        hexWatcher.enable(hexEnabled);
        sendText.addTextChangedListener(hexWatcher);
        sendText.setHint(hexEnabled ? "HEX mode" : "");

        View sendBtn = view.findViewById(R.id.send_btn);
        sendBtn.setOnClickListener(v -> send(sendText.getText().toString()));
        return view;
    }

    @Override
    public void onCreateOptionsMenu(@NonNull Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.menu_terminal, menu);
        menu.findItem(R.id.hex).setChecked(hexEnabled);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.clear) {
            receiveText.setText("");
            return true;
        } if (id == R.id.plot){
            Intent intent = new Intent(getActivity(), MainActivity2.class);
            startActivity(intent);
            return true;
        }else if (id == R.id.newline) {
            String[] newlineNames = getResources().getStringArray(R.array.newline_names);
            String[] newlineValues = getResources().getStringArray(R.array.newline_values);
            int pos = java.util.Arrays.asList(newlineValues).indexOf(newline);
            AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
            builder.setTitle("Newline");
            builder.setSingleChoiceItems(newlineNames, pos, (dialog, item1) -> {
                newline = newlineValues[item1];
                dialog.dismiss();
            });
            builder.create().show();
            return true;
        } else if (id == R.id.hex) {
            hexEnabled = !hexEnabled;
            sendText.setText("");
            hexWatcher.enable(hexEnabled);
            sendText.setHint(hexEnabled ? "HEX mode" : "");
            item.setChecked(hexEnabled);
            return true;
        } else {
            return super.onOptionsItemSelected(item);
        }
    }

    /*
     * Serial + UI
     */
    private void connect() {
        try {
            BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
            BluetoothDevice device = bluetoothAdapter.getRemoteDevice(deviceAddress);
            status("connecting...");
            connected = Connected.Pending;
            SerialSocket socket = new SerialSocket(getActivity().getApplicationContext(), device);
            service.connect(socket);
        } catch (Exception e) {
            onSerialConnectError(e);
        }
    }

    private void disconnect() {
        connected = Connected.False;
        service.disconnect();
    }

    private void send(String str) {
        if(connected != Connected.True) {
            Toast.makeText(getActivity(), "not connected", Toast.LENGTH_SHORT).show();
            return;
        }
        try {
            String msg;
            byte[] data;
            if(hexEnabled) {
                StringBuilder sb = new StringBuilder();
                TextUtil.toHexString(sb, TextUtil.fromHexString(str));
                TextUtil.toHexString(sb, newline.getBytes());
                msg = sb.toString();
                data = TextUtil.fromHexString(msg);
            } else {
                msg = str;
                data = (str + newline).getBytes();
            }
            SpannableStringBuilder spn = new SpannableStringBuilder(msg + 'n');
            spn.setSpan(new ForegroundColorSpan(getResources().getColor(R.color.colorSendText)), 0, spn.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
            receiveText.append(spn);
            service.write(data);
        } catch (Exception e) {
            onSerialIoError(e);
        }
    }

    private void receive(byte[] data) {
        if(hexEnabled) {
            receiveText.append("Hello" + TextUtil.toHexString(data) + 'n');
        } else {
            String msg = new String(data);
            if(newline.equals(TextUtil.newline_crlf) && msg.length() > 0) {
                // don't show CR as ^M if directly before LF
                msg = msg.replace(TextUtil.newline_crlf, TextUtil.newline_lf);
                // special handling if CR and LF come in separate fragments
                if (pendingNewline && msg.charAt(0) == 'n') {
                    Editable edt = receiveText.getEditableText();
                    if (edt != null && edt.length() > 1)
                        edt.replace(edt.length() - 2, edt.length(), "");
                }
                pendingNewline = msg.charAt(msg.length() - 1) == 'r';
            }
            receiveText.append(TextUtil.toCaretString(msg, newline.length() != 0)); //print out data
        }
    }

    private void status(String str) {
        SpannableStringBuilder spn = new SpannableStringBuilder(str + 'n');
        spn.setSpan(new ForegroundColorSpan(getResources().getColor(R.color.colorStatusText)), 0, spn.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
        receiveText.append(spn);
    }

    /*
     * SerialListener
     */
    @Override
    public void onSerialConnect() {
        status("connected");
        connected = Connected.True;
    }

    @Override
    public void onSerialConnectError(Exception e) {
        status("connection failed: " + e.getMessage());
        disconnect();
    }

    @Override
    public void onSerialRead(byte[] data) {
        receive(data);
    }

    @Override
    public void onSerialIoError(Exception e) {
        status("connection lost: " + e.getMessage());
        disconnect();
    }

}

MainActivity2.java defines activity_main2.xml

package de.kai_morich.simple_bluetooth_le_terminal;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;

import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.Toast;

public class MainActivity2 extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        Toolbar toolbar = findViewById(R.id.toolbar2);
        setSupportActionBar(toolbar);
        

        Intent intent = getIntent();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu_plot,menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.dataplot:
                Toast.makeText(this, "dataplot", Toast.LENGTH_SHORT).show();
                return true;
            case R.id.fft:
                Toast.makeText(this, "FFT", Toast.LENGTH_SHORT).show();
                return true;
            case R.id.data:
                Toast.makeText(this, "DATA", Toast.LENGTH_SHORT).show();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }
}

activity_main2.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity2">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar2"
        android:layout_width="409dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="1dp"
        android:layout_marginEnd="1dp"
        android:layout_marginBottom="675dp"
        android:background="?attr/colorPrimary"
        android:minHeight="?attr/actionBarSize"
        android:theme="?attr/actionBarTheme"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer

I think your Toolbar is pushed out of the screen by the layout_marginBottom = 675dp. If you want to show the Toolbar at top of the screen I would suggest this:

 <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar2"
        android:layout_width="0dp"
        android:layout_height="?attr/actionBarSize"
        android:layout_marginStart="1dp"
        android:layout_marginEnd="1dp"
        android:background="?attr/colorPrimary"
        android:theme="?attr/actionBarTheme"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />