Android : Android: member variable becoming null

I have an activity - MainActivity, where I initialize member variable in onCreate(...) method. This variable is then used in onStart() and onStop() methods.

Pretty simple, huh? Now the issue is, that from my users, I started to get NullPointerException crash reports, which happens in onStop() method.

I've been extensively searching for the cause, and I understand that static variables may become null when android decides to free up memory, however I couldn't find a case with member variable, which is initialized in onCreate(). The code is following:

public class MainActivity extends ActionBarActivity implements ActionBar.TabListener, SharedPreferences.OnSharedPreferenceChangeListener {
private MySvc mySvc;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);     
    mySvc = DI.i().getMySvc();
    ...
}

@Override
protected void onStart() {
    super.onStart();
    //here I get NPE
    mySvc.start();
}

...

@Override
protected void onStop() {
    super.onStop();
    mySvc.start();
}
}

}

To make things even more mysterious, I started to get this crash reports only from certain versions of app, however the old versions have same behavior in onStart() and onStop().

I will be grateful for any hint.