Fixed crash when next/previous button is pressed

when current song is about to finish.

When the user wants to play a different track in
this case when next/previous is triggered, player
should be set back to NULL state, then the
uri property should be set to the new location
and then player be set to PLAYING state again.

Fixes #375
2 jobs for iss375 in 7 minutes and 13 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Check
passed #642688
flake8

00:01:49

 
  Build Flatpak
passed #642689
flatpak
flatpak

00:05:23