AppCompat does not support the current theme features - nothing works
Change:
<item name="android:windowNoTitle">true</item>
to:
<item name="windowNoTitle">true</item>
To anyone else having this problem, I just found the problem: Apparently there exists another theme with name "AppTheme". I don't know why and how, but changing my theme's name solved the problem.
This was the answer to my problem.