Why is my App crashing on start? android

Question

I am pretty new to coding, so I have no idea what the errormessage is trying to tell me, also a GoogleSearch didn't really help me.

Here is the javaCode that I wrote:

Button   mButton;
    EditText mEdit;
    Button testluck;
    EditText nummer;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        testluck = (Button)findViewById(R.id.testluck);
        mButton = (Button)findViewById(R.id.okay);
        mEdit  = (EditText)findViewById(R.id.name);
        final TextView questionOne =(TextView)findViewById(R.id.questionOne);
        final String name = mEdit.getText().toString();
        final TextView textfield =(TextView)findViewById(R.id.textfield);
        final TextView deineNummer =(TextView)findViewById(R.id.deineNummer);
        nummer = (EditText)findViewById(R.id.name);
        String nummerstring = nummer.getText().toString();
        final int nummereins =Integer.parseInt(nummerstring);
        final TextView unterschied =(TextView)findViewById(R.id.unterschied);

        mButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                String name = mEdit.getText().toString();
                questionOne.setText("Lass uns ein Spiel spielen, " + name + "!");
                mEdit.setText("");
                textfield.setText("Was ist deine Glückszahl zwischen 1 und 100?");
                mButton.setText("");
            }
        });

        testluck.setOnClickListener(new View.OnClickListener(){
            public void onClick(View v){
                deineNummer.setText("Deine gewählte Nummer ist " + nummereins);
                int Gewinnzahl = (int) (Math.random()*100 +1);
                int Unterschied = Math.abs(Gewinnzahl - nummereins);
                unterschied.setText(Unterschied);
            }
        });
    }
}

and when I try to run the app it crashes, showing me this ErrorLog:

FATAL EXCEPTION: main
Process: com.example.android.testyourluck, PID: 4829
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.testyourluck/com.example.android.testyourluck.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
    at android.app.ActivityThread.access$800(ActivityThread.java:135)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:136)
    at android.app.ActivityThread.main(ActivityThread.java:5017)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
    at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
    at com.example.android.testyourluck.MainActivity.onCreate(MainActivity.java:32)
    at android.app.Activity.performCreate(Activity.java:5231)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
    at android.app.ActivityThread.access$800(ActivityThread.java:135) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:136) 
    at android.app.ActivityThread.main(ActivityThread.java:5017) 
    at java.lang.reflect.Method.invokeNative(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:515) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
    at dalvik.system.NativeStart.main(Native Method) 

Can someone please tell me what I have done wrong? I would also love to hear how I could optimize my code!