مشکل FATAL EXCEPTION

سلام و عرض تبریک

در پروژه‌ی ماشین حساب هنگام پیاده سازی دکمه back به مشکلی برخوردم. به سایت stackoverflow هم مراجعه کردم اما مشکل حل نشد. در قسمت مانیتورینگ اندروید این عبارت درج میشه:

 FATAL EXCEPTION: main
                                                 java.lang.RuntimeException: Unable to start activity ComponentInfo{--------------------.kmapp.MainActivity}: java.lang.NullPointerException
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
                                                     at android.app.ActivityThread.access$600(ActivityThread.java:130)
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
                                                     at android.os.Handler.dispatchMessage(Handler.java:99)
                                                     at android.os.Looper.loop(Looper.java:137)
                                                     at android.app.ActivityThread.main(ActivityThread.java:4745)
                                                     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:786)
                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
                                                     at dalvik.system.NativeStart.main(Native Method)
                                                  Caused by: java.lang.NullPointerException
                                                     at ----------------.kmapp.MainActivity.onCreate(MainActivity.java:241)
                                                     at android.app.Activity.performCreate(Activity.java:5008)
                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
                                                     at android.app.ActivityThread.access$600(ActivityThread.java:130) 
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
                                                     at android.os.Handler.dispatchMessage(Handler.java:99) 
                                                     at android.os.Looper.loop(Looper.java:137) 
                                                     at android.app.ActivityThread.main(ActivityThread.java:4745) 
                                                     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:786) 
                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
                                                     at dalvik.system.NativeStart.main(Native Method) 

کد دکمه:

back.setOnClickListener(new View.OnClickListener() {
  @Override
  public void onClick(View v) {
    back();
  }
});

متود back:

public void  back() {
  String value = natije.getText().toString();
  String newValue = value.substring(0, value.length()-1);
  natije.setText(newValue);
}

بسیار ممنونم از اینکه وقت می‌گذارید.