Fix settings manager assignment

[ReneLu]

From Git Rebase Release of 0.0.1-alpha StreamController/StreamController-Store#108 (comment)

What I noticed though, that directly after installation I got this AttributeError: 'SpotifyControl' object has no attribute '_settings_manager' error when trying to open the plugin settings in l.109 of your main.py.
After a restart of the app this error was gone, but instead I got an error because an empty string (the stored port) couldn't be converted into an int.