Fixed a small crash possibility

app/src/main/java/de/kuschku/util/servicebound/BoundFragment.java

@@ -79,6 +79,7 @@ public abstract class BoundFragment extends Fragment {
     @Override
     public void onStop() {
         super.onStop();
+        if (binder != null)
             binder.removeCallback(consumer);
         ServiceHelper.disconnect(getContext(), connection);
     }