I want to transition the screen from Main to Category

When you click the "Start" button on the Main screen, the application will end and the following error message will be displayed.

unfortunately, QuestionApplication has stopped.

activity_main.xml

<? xml version = "1.0" encoding = "utf-8"?><TextView
        android: id = "@ + id/textPage"
        android: layout_width = "wrap_content"
        android: layout_height = "wrap_content"
        android: text = "Main"
        android: textSize = "30sp"
        app: layout_constraintLeft_toLeftOf = "parent"
        app: layout_constraintTop_toTopOf = "parent" /><Button
        android: id = "@ + id/buttonStart"
        android: layout_width = "250sp"
        android: layout_height = "130sp"
        android: text = "start"
        android: textSize = "50sp"
        app: layout_constraintTop_toTopOf = "parent"
        app: layout_constraintBottom_toBottomOf = "parent"
        app: layout_constraintLeft_toLeftOf = "parent"
        app: layout_constraintRight_toRightOf = "parent" /></androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

package com.example.questionapplication;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;import android.widget.Button;
import android.content.Intent;
public class MainActivity extends AppCompatActivity {
    Button btnS;
    @Override
    protected void onCreate (Bundle savedInstanceState) {
        super.onCreate (savedInstanceState);
        setContentView (R.layout.activity_main);
        btnS = (Button) findViewById (R.id.buttonStart);// Start button
        btnS.setOnClickListener (new View.OnClickListener () {
            public void onClick (View view) {
                Intent intent = new Intent ();
                intent.setClassName ("com.example.questionapp", "com.example.questionapp.CategoryActivity");
                startActivity (intent);
            }
        });
    }
}

activity_category.xml

<? xml version = "1.0" encoding = "utf-8"?><TextView
        android: id = "@ + id/textPage"
        android: layout_width = "wrap_content"
        android: layout_height = "wrap_content"
        android: text = "Category"
        android: textSize = "30sp"
        app: layout_constraintLeft_toLeftOf = "parent"
        app: layout_constraintTop_toTopOf = "parent" /></androidx.constraintlayout.widget.ConstraintLayout>

CategoryActivity.java

package com.example.questionapplication;
import androidx.appcompat.app.AppCompatActivity;import android.os.Bundle;
import android.widget.TextView;
public class CategoryActivity extends AppCompatActivity {
    TextView txtP;
    @Override
    protected void onCreate (Bundle savedInstanceState) {
        super.onCreate (savedInstanceState);
        setContentView (R.layout.activity_category);
        txtP = (TextView) findViewById (R.id.textPage);
    }
}

AndroidManifest.xml

<? xml version = "1.0" encoding = "utf-8"?><manifest xmlns: android = "http://schemas.android.com/apk/res/android"
    package = "com.example.questionapplication">         </activity>                <intent-filter>                            <category android: name = "android.intent.category.LAUNCHER" />        </intent-filter>    </activity></application></manifest>

Re-enter and confirm AndroidManifest.xml
AVD Manager Data Erase

Android Studio 4.1
AVD: Nexus7 (2012) API23
SDK: Android 6.0 (Marshmallow) API level 23