Anfängerproblem: ImageView Initialisierung wirft NullPointerException auf

Haudiho,
ich bin ziemlich frisch in der Androidprogramierung und habe folgendes Problem:
In meinem Programm wirft die Initialisierung eines ImageViews eine NullPointerException auf.
Ich hab leider sowohl keine Ahnung was das heißt, noch was ich da falsch gemacht habe.
Hier ist mein Code:

import android.media.MediaPlayer;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.ImageView;

public class GameActivity extends AppCompatActivity implements View.OnClickListener {

int counter = 0;
int round = 1;
int coincidence = 0;
boolean finished = true;
final ImageView gameImages = (ImageView)findViewById(R.id.gameImageView);
final MediaPlayer mediaPlayer1 = MediaPlayer.create(GameActivity.this, R.raw.sound1);
final MediaPlayer mediaPlayer2 = MediaPlayer.create(GameActivity.this, R.raw.sound2);
final MediaPlayer mediaPlayer3 = MediaPlayer.create(GameActivity.this, R.raw.sound3);
final MediaPlayer mediaPlayer4 = MediaPlayer.create(GameActivity.this, R.raw.sound4);
final MediaPlayer mediaPlayer5 = MediaPlayer.create(GameActivity.this, R.raw.sound5);
final MediaPlayer mediaPlayer6 = MediaPlayer.create(GameActivity.this, R.raw.sound6);

@Override
protected void onCreate(Bundle savedInstanceState) {


    super.onCreate(savedInstanceState);
    setContentView(R.layout.game_activity);
    gameImages.setOnClickListener(GameActivity.this);
}

public void onClick(View v) {
    counter++;
    coincidence = (int) (Math.random() * 6 + 1);
    if (finished) {
        switch (coincidence) {
            case 1:
                mediaPlayer1.start();
            case 2:
                mediaPlayer2.start();
            case 3:
                mediaPlayer3.start();
            case 4:
                mediaPlayer4.start();
            case 5:
                mediaPlayer5.start();
            case 6:
                mediaPlayer6.start();
        }
    }
    showImages();


}

 public void showImages(){
    if (counter >= round*4){
        gameImages.setImageResource(R.drawable.picture3);
        finished = false;

    }
    else{
        if (counter >= round*3){
            gameImages.setImageResource(R.drawable.picture2);

        }
        else{
            if (counter >= round*2){
                gameImages.setImageResource(R.drawable.picture1);
            }
            else{
                if (counter >= round*1){
                    gameImages.setImageResource(R.drawable.picture0);
                }
        }
    }
}

}

Ich habe den Code ein bisschen abgekürzt, bei mir zeigt er nach und nach 43 Bilder an.

Hier ist die Fehlermeldung, die die Ausführung der App aufwürft:

> 05-10 18:54:04.141 3589-3589/slartbart.example E/AndroidRuntime: FATAL EXCEPTION: main
                                                                Process: slartbart.example, PID: 3589
                                                                java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{slartbart.punchanazi/slartbart.example.GameActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                                                                    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567)
                                                                    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
                                                                    at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                                    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
                                                                    at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                    at android.os.Looper.loop(Looper.java:154)
                                                                    at android.app.ActivityThread.main(ActivityThread.java:6119)
                                                                    at java.lang.reflect.Method.invoke(Native Method)
                                                                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                                                                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
                                                                 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
                                                                    at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120)
                                                                    at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:151)
                                                                    at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31)
                                                                    at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55)
                                                                    at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33)
                                                                    at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:33)
                                                                    at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
                                                                    at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
                                                                    at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:525)
                                                                    at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:193)
                                                                    at slartbart.example.GameActivity.<init>(GameActivity.java:18)
                                                                    at java.lang.Class.newInstance(Native Method)
                                                                    at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
                                                                    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557)
                                                                    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
                                                                    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                                    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
                                                                    at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                    at android.os.Looper.loop(Looper.java:154) 
                                                                    at android.app.ActivityThread.main(ActivityThread.java:6119) 
                                                                    at java.lang.reflect.Method.invoke(Native Method) 
                                                                    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                                                                    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

Könnt ihr mir bitte sagen, was der Fehler genau bedeutet und wie ich ihn vermeiden bzw. beheben kann?
Mit freundlichen Grüßen,
Ford.

Vielen Dank, Problem gelöst