I just tried your App and it looks good. I wondered that it doesn't request for any access permissions. Which is good. Unfortunately it also does not do anything. After the timer expired nothing happened. What have I been doing wrong? My Android version is 7.0.
I have to apologize. The app works like a charm. The problem was, that with Spotify I played the music over a different device (my Yamaha AV receiver), which unfortunately is not muted after the timer expires. But if it is played over the Smartphone itself, it works like a charm.
I mean I do have an iPhone but I have seen several people reply to the thread with solutions for android users as well. The solutions have been great help either way and I have definitely been using the suggested sleep timer on my iPhone. I will say though it would still be nice if Spotify could just make that a built in feature so that android users don’t have to download a whole other app just to be able to have the ability to put the music on a timer.