NullPointerException with Otto and Dagger

I'm new to Otto and I'm having serious trouble making out its functioning together with Dagger dependency injector and JobManager. Whenever I launch my application, I keep getting the same error message:

"java.lang.RuntimeException: Unable to resume activity {sdos.juanjosemelero.pruebaormlite2/sdos.juanjosemelero.pruebaormlite2.MainActivity}:
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.squareup.otto.Bus.register(java.lang.Object)' on a null object reference"

[...]

"Caused by java.lang.NullPointerException:
Attempt to invoke virtual method 'void com.squareup.otto.Bus.register(java.lang.Object)' on a null object reference".

The line it reffers is this one:

bus.register(this);

This is my activity:

public class MainActivity extends ActionBarActivity {

    @Inject Bus bus;   

    @Override
    protected void onResume() {
        super.onResume();
        bus.register(this);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.inject(this);

    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        bus.unregister(this);
    }
}

And here is my Module for Dagger injection:

@Module(
    injects = {MainActivity.class},
    library = true
)

public class MyModule {

    private final MyApplication application;

    public MyModule (MyApplication application) {
        this.application = application;
    }

    @Provides
    @Singleton
    MyApplication provideMyApplication() {
        return application;
    }

    @Provides
    @Singleton
    public Bus provideBus (){
        return new Bus(ThreadEnforcer.ANY);
    }
}

And MyApplication class in case it is interesting to take a look at it:

public class MyApplication extends Application {

    private ObjectGraph objectGraph;
    private static MyApplication application;

    //Constructor
    public MyApplication() {
        application = this;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        buildObjectGraph();
    }

    public void buildObjectGraph() {
        objectGraph = ObjectGraph.create(new MyModule(this));
    }

    public void inject(Object o) {
        objectGraph.inject(o);
    }

    public static MyApplication get() {
        return application;
    }
}

Am I suppose to initialize MainActivity somehow?

You're not injecting the MainActivity in onCreate with the object graph. That's all.

Just call

MyApplication.get().inject(this);

in MainActivity onCreate().