Crash on WebSocketService#onDestroy

[jmattheis]

From the Google play console:

Exception java.lang.RuntimeException:
  at android.app.ActivityThread.handleStopService (ActivityThread.java:4900)
  at android.app.ActivityThread.-$$Nest$mhandleStopService
  at android.app.ActivityThread$H.handleMessage (ActivityThread.java:2264)
  at android.os.Handler.dispatchMessage (Handler.java:106)
  at android.os.Looper.loopOnce (Looper.java:210)
  at android.os.Looper.loop (Looper.java:299)
  at android.app.ActivityThread.main (ActivityThread.java:8252)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:559)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:954)
Caused by java.lang.IllegalArgumentException: NetworkCallback was not registered
  at android.net.ConnectivityManager.unregisterNetworkCallback (ConnectivityManager.java:4979)
  at com.github.gotify.service.WebSocketService.onDestroy (WebSocketService.kt:83)
  at android.app.ActivityThread.handleStopService (ActivityThread.java:4880)

Likely introduced in #288 not sure yet why this is happening yet.