Android test with Espresso IdlingResource

I'm trying to test that the AutoCompleteTextView will show the items after some word will be typed. But there is a delay between typing and showing the popup. First i was using Thread.sleep() and it was working just fine. But I know that this approach isn't clear so I'm trying to accomplish it with IdlingResource. But it doesn't work for me. I literally read first 5 pages of Google responses, but either I don't understand how it should work, or I have some error in my code.

Here is the code:

static class AutocompleteShowIdlingResource implements IdlingResource {

    private Activity activity;
    private @IdRes int resId;
    private ResourceCallback resourceCallback;

    public AutocompleteShowIdlingResource(Activity activity, @IdRes int resId) {
        this.activity = activity;
        this.resId = resId;
    }

    @Override
    public String getName() {
        return this.getClass().getName() + resId;
    }

    @Override
    public boolean isIdleNow() {
        boolean idle = ((AutoCompleteTextView) activity.findViewById(resId)).getAdapter() != null;
        Log.d(TAG, "isIdleNow: " + idle);
        if (idle) {
            resourceCallback.onTransitionToIdle();
        }
        return idle;
    }

    @Override
    public void registerIdleTransitionCallback(ResourceCallback callback) {
        this.resourceCallback = callback;

    }
}

The test itself:

    onView(withId(R.id.autocomplete_occupation)).perform(typeText("dok"));
    IdlingResource idlingResource = new AutocompleteShowIdlingResource(activity, R.id.autocomplete_occupation);
    Espresso.registerIdlingResources(idlingResource);
    assertEquals(((AutoCompleteTextView) activity.findViewById(R.id.autocomplete_occupation)).getAdapter().getCount(), 3);
    Espresso.unregisterIdlingResources(idlingResource);

But the test fails on java.lang.NullPointerException when trying to call getCount() on null adapter. The log is printing

isIdleNow: false

just once, which is quite strange.

There isn't much clear examples how to use IdlingResource, so maybe someone can make it clear for me. Thanks.