Why does the app crash when switching to another activity?

Question

Java

'Notes' application
When USB debugging starts, main_activity opens, and when you press the '+' button, the application closes.
I've already broken my head, what's wrong ....
Help me find the error, please.

MainActivity:

package com.example.testing_interface.screens;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
import androidx.recyclerview.widget.DividerItemDecoration;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import com.example.testing_interface.R;
import com.example.testing_interface.model.Note;
import com.example.testing_interface.screens.detals.NoteDetailsActivity;
import java.util.List;


public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        recyclerView = findViewById(R.id.list);
        LinearLayoutManager linearLayoutManager = new  LinearLayoutManager(this, RecyclerView.VERTICAL, false);
        recyclerView.setLayoutManager(linearLayoutManager);
        recyclerView.addItemDecoration(new DividerItemDecoration(this, DividerItemDecoration.VERTICAL));

        final Adapter adapter = new Adapter();
        recyclerView.setAdapter(adapter);

        Button add_button = (Button) findViewById(R.id.button_add);
        add_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NoteDetailsActivity.start(MainActivity.this, null);
            }
        });

        MainViewModel mainViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
        mainViewModel.getNoteLiveData().observe(this, new Observer<List<Note>>() {
            @Override
            public void onChanged(List<Note> notes) {
                adapter.setItems(notes);
            }
        });
    }
}

activity_main.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"
    android:background="@color/mainBgColor"
    tools:context=".screens.MainActivity">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@+id/constraintLayout"
        android:layout_width="50dp"
        android:layout_height="0dp"
        android:background="@color/sideBgColor"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <Button
            android:id="@+id/button_add"
            android:layout_width="40dp"
            android:layout_height="40dp"
            android:layout_marginBottom="400dp"
            android:background="@android:drawable/ic_input_add"
            android:gravity="center"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />
    </androidx.constraintlayout.widget.ConstraintLayout>

    <TextView
        android:id="@+id/notes_header"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="40dp"
        android:layout_marginLeft="40dp"
        android:layout_marginTop="40dp"
        android:text="@string/app_headle"
        android:textColor="@android:color/background_dark"
        android:textSize="40sp"
        app:layout_constraintStart_toEndOf="@+id/constraintLayout"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/list"
        android:layout_width="359dp"
        android:layout_height="563dp"
        android:layout_marginStart="40dp"
        android:layout_marginLeft="40dp"
        android:layout_marginTop="30dp"
        android:layout_marginEnd="40dp"
        android:layout_marginRight="40dp"
        android:layout_marginBottom="30dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/constraintLayout"
        app:layout_constraintTop_toBottomOf="@+id/notes_header" />

</androidx.constraintlayout.widget.ConstraintLayout>

NoteDetailActivity (the form into which the transition occurs when creating a new note (and crashes)):

package com.example.testing_interface.screens.detals;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.MenuItem;
import android.widget.EditText;
import android.widget.Toolbar;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import com.example.testing_interface.App;
import com.example.testing_interface.R;
import com.example.testing_interface.model.Note;

public class NoteDetailsActivity extends AppCompatActivity {

    private static final String EXTRA_NOTE = "NoteDetailsActivity.EXTRA_NOTE";

    private Note note;
    private EditText editText;

    public static void  start(Activity caller, Note note){
        Intent intent = new Intent(caller, NoteDetailsActivity.class);
        if(note != null){
            intent.putExtra(EXTRA_NOTE, note);
        }
        caller.startActivity(intent);
    }
    private void setSupportActionBar(Toolbar toolbar) {
    }

    Toolbar toolbar = findViewById(R.id.toolbar);

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_note_details);

        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setHomeButtonEnabled(true);

        setTitle(getString(R.string.note_details_title));

        editText = findViewById(R.id.text);

        if(getIntent().hasExtra(EXTRA_NOTE)){
            note = getIntent().getParcelableExtra(EXTRA_NOTE);
            editText.setText(note.text);
        } else {
            note = new Note();
        }

    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId())
        {
            case android.R.id.home:
                finish();
                break;
            case R.id.action_save:
                if(editText.getText().length() > 0){
                    note.text = editText.getText().toString();
                    note.done = false;
                    note.timestump = System.currentTimeMillis();
                    if(getIntent().hasExtra(EXTRA_NOTE)){
                        App.getInstance().getNoteDao().update(note);
                    } else {
                        App.getInstance().getNoteDao().insertAll(note);
                    }
                    finish();
                }
                break;
        }
        return super.onOptionsItemSelected(item);
    }
}

activity_note_details.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"
    android:background="@color/mainBgColor">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="@color/sideBgColor"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="10dp"
        android:layout_marginLeft="10dp"
        android:layout_marginEnd="10dp"
        android:layout_marginRight="10dp"
        android:hint="@string/note_text"
        android:minHeight="200dp"
        android:gravity="top"
        android:padding="16dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/toolbar" />

    <Button
        android:id="@+id/action_save"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:layout_marginTop="3dp"
        android:layout_marginEnd="10dp"
        android:layout_marginRight="10dp"
        android:layout_marginBottom="8dp"
        android:background="@android:drawable/ic_menu_save"
        app:layout_constraintBottom_toBottomOf="@+id/toolbar"
        app:layout_constraintEnd_toEndOf="@+id/toolbar"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

look in the manifest - is the second activity registered?

Answer 2

You NoteDetailsActivityseem to crash in this place:
setSupportActionBar(toolbar);
Reason 1:
Toolbar toolbar = findViewById(R.id.toolbar);
This field must be initialized in the method onCreate()and must be initialized after the call setContentView(), otherwise it findViewById()returns null.
Reason 2:

private void setSupportActionBar(Toolbar toolbar) {
    }

I don't know why you have this method of the parent class overridden, maybe by accident.
You need to either remove it completely, or call a method from the parent class in it:

private void setSupportActionBar(Toolbar toolbar) {
    super.setSupportActionBar(toolbar);
}