NullPointerException when mocking SharedPreferences or Application

I have a subclass of Application like so:

public class AppState extends Application {
    private static AppState instance;
    private static Context context;

    public static void injectContextAndInstanceForTesting(Context context, AppState instance) {
        AppState.instance = instance;
        AppState.context = context;
    }
}

I create some mocks with:

mockSharedPrefs = mock(SharedPreferences.class);
mockAppState = mock(AppState.class);
mockContext = mock(Context.class);
when(mMockContext.getSharedPreferences(anyString(), anyInt())).thenReturn(mockSharedPrefs);

AppState.injectContextAndInstanceForTesting(mMockContext, mockAppState);

At this point I run my test and get a NullPointerException when the code hits

SharedPreferences.edit().putString();

Test failed to run to completion. 
Reason: 'Instrumentation run failed due to 'java.lang.NullPointerException''. 
Check device logcat for details
Tests on Nexus5 - 4.4.4 failed: Instrumentation run failed due to 'java.lang.NullPointerException'

Does anyone have experience mocking out SharedPreferences and/or Application? I'm not sure which of these is throwing the exception, as the unit test is running in an automated test harness that doesn't have access to the logcat.