App crashes with error "Unfortunately MyApp has stopped". How to fix?

Hi everyone. I'm a beginner in Android development in Java. I created a simple app — a calculator with two input fields and an "Add" button. When running on the emulator, everything works, but on a real phone (Android 9) when clicking the button, the app crashes with the message "Unfortunately MyApp has stopped". Here is my code in MainActivity.java: ```java public class MainActivity extends AppCompatActivity { EditText num1, num2; Button addBtn; TextView result; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); num1 = findViewById(R.id.num1); num2 = findViewById(R.id.num2); addBtn = findViewById(R.id.addBtn); result = findViewById(R.id.r