我只是想在Mainactivity中使用2个按钮。一个打开一个网站（有效），另一个打开我的第二堂课（为简化起见，我仅设置为第二个网站）。但是，尽管按钮设置类似于其他按钮，但使用setOnClicklistener以及资源中定义的变量，但我收到以下错误消息。 我已经排除了几个片段-错误消息似乎与访问第二类的按钮有关。

为那些琐碎的事情而致歉...我迷失了，但为任何暗示而感激。

java.lang.RuntimeException: Unable to start activity ComponentInfo{paperpad.app/paperpad.app.Mainactivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClicklistener(android.view.View$OnClicklistener)' on a null object reference

strings.xml

<resources>
    <string name="logo">Logo</string>
    <string name="app_name">PaperPad2</string>
    <string name="button_go2activity">Start Drawing</string>
    <string name="button_openPicture">Load Picture</string>
    <string name="button_closePicture">Stop</string>
    <string name="button_help">help</string>
    <string name="websiteAddress">PaperPad.app</string>
</resources>

Mainactivity.java

package paperpad.app;
        import androidx.annotation.NonNull;
//etc.

public class Mainactivity extends AppCompatactivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button button_openHelp = findViewById(R.id.button_openHelp);    
        button_openHelp.setOnClicklistener(new View.OnClicklistener() {     
            @Override
            public void onClick(View view) {                  
                Intent openHelp = new Intent(Intent.actION_VIEW,Uri.parse("http://www.youtube.com"));     
                startactivity(openHelp);
            }
        });
        Button button_go2activity = findViewById(R.id.button_go2activity);    
        button_go2activity.setOnClicklistener(new View.OnClicklistener() {     
            @Override
            public void onClick(View view) {                  
                Intent openLink = new Intent(Intent.actION_VIEW,Uri.parse("http://www.google.com"));      
                startactivity(openLink);
            }
        });
    }
}

我的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:id="@+id/parentLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:clickable="false"
    tools:context=".Mainactivity">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="254dp"
        android:layout_height="237dp"
        android:layout_marginTop="48dp"
        android:layout_marginBottom="8dp"
        android:contentDescription="@string/logo"
        app:layout_constraintBottom_toTopOf="@+id/imageView2"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@drawable/logo_paperpad" />

    <ImageView
        android:id="@+id/imageView2"
        android:layout_width="wrap_content"
        android:layout_height="59dp"
        android:layout_marginTop="96dp"
        android:layout_marginBottom="100dp"
        app:layout_constraintBottom_toTopOf="@+id/button_openPicture"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/imageView"
        app:srcCompat="@drawable/logo_titel"
        tools:ignore="ContentDescription" />

    <Button
        android:id="@+id/button_go2activity"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="269dp"
        android:text="@string/button_go2activity"
        android:textSize="24sp"
        android:typeface="normal"
        app:layout_constraintBottom_toTopOf="@+id/button_openHelp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/imageView2" />

    <Button
        android:id="@+id/button_openHelp"
        android:layout_width="45dp"
        android:layout_height="25dp"
        android:layout_marginBottom="10dp"
        android:background="#FFFFFF"
        android:text="@string/button_help"
        android:textColor="@android:color/black"
        android:textSize="12sp"
        app:layout_constraintBottom_toTopOf="@+id/linkWebsite"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button_go2activity" />


    <TextView
        android:id="@+id/linkWebsite"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:inputType="none"
        android:text="@string/websiteAddress"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button_openHelp" />


</androidx.constraintlayout.widget.ConstraintLayout>