RunTimeException in Android espresso when selecting spinner in dialog

I had the same error when I tried to match an item inside a spinner inside a dialog-fragment. This helped:

onView(withText(value)).inRoot(isPlatformPopup()).perform(click());

Try this:

onData(allOf(is(instanceOf(String.class)), is(value)))
    .inRoot(isPlatformPopup()).perform(click());