RadioGroup.setOnCheckedChangeListener(this) causes NullPointerException

Question

I am from Germany and new in programming with java and Android. In my first App, I have 2 Pages. The first page (aktivity_main.xml) is empty at this moment. There, I want to build a listView. when I go in the menu and click on "Hinzufügen" (--> means "Add" in english), the second page (mypage) opens.

In this, I can type in my Inputs. And here starts my Problem. (I searched for days but however I do it, it doesnt work!) When I change the state in the radiogroup, the Text (TextDefinitionWert: "Bitte geben Sie Ihren gewünschten Maximalwert ein:") should change in anohter Text, saved in String.xml. But it doesnt work. There is only a NullPointerException

This is my code for MainActivity.Java:

package ilyas.ausgabenkontrolle;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.*;
import android.widget.RadioGroup.*;



public class MainActivity extends Activity implements RadioGroup.OnCheckedChangeListener {

TextView TextHeaderNeuBearbeiten;
TextView TextDefinitionWert;
RadioButton RadioUp;
RadioButton RadioDown;
EditText EditName;
EditText EditWert;
Button ButtonOKnb;
RadioGroup RadioGroup;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    TextHeaderNeuBearbeiten = (TextView) findViewById(R.id.TextHeaderNeuBearbeiten) ;
    TextDefinitionWert = (TextView ) findViewById(R.id.TextDefinitionWert);
    RadioUp = (RadioButton) findViewById(R.id.RadioUp);
    RadioDown = (RadioButton) findViewById(R.id.RadioDown);
    EditName = (EditText) findViewById(R.id.EditName);
    EditWert = (EditText) findViewById(R.id.EditWert);
    ButtonOKnb = (Button) findViewById(R.id.ButtonOKnb);
    RadioGroup = (RadioGroup) findViewById(R.id.radioGroup1);
//the next line causes the Exception. When I delete this line, the Emulator starts, but when I change the stats, nothing happens:
    RadioGroup.setOnCheckedChangeListener(this);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.activity_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (item.toString().equals("Hinzufügen")) {
        setContentView(R.layout.neuesbearbeiten);
    }
    return true;
}


public void onCheckedChanged(RadioGroup RadioGroup, int checkedId) {
    // TODO Auto-generated method stub
switch (checkedId) {
    case R.id.RadioUp:
        TextDefinitionWert.setText(R.string.definitionup);
        break;
    case R.id.RadioDown:
        TextDefinitionWert.setText(R.string.definitiondown);
        break;
    }
}

}

(Because of I am new here, i cannot post Images.)

The LogCat is like this:

ilyas.ausgabenkontrolle/ilyas.ausgabenkontrolle.MainActivity}: java.lang.NullPointerException
at android.app.activityThread.performLaunchActivity(ActivityThread.java:1956)
at android.app.activityThread.handleLaunchActivity(ActivityThread.java:1981)
at android.app.activityThread.access$600(ActivityThread.java:123)
at android.app.activityThread$H.handleMessage(ActivityThread.java:1147)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4424)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at ilyas.ausgabenkontrolle.MainActivity.onCreate(MainActivity.java:37)
at android.app.Activity.performCreate(Activity.java:4465)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
at android.app.activityThread.performLaunchActivity(ActivityThread.java:1920)
... 11 more

I hope You can help me. I dont know why this is happening.