Attempt to fix locationManager crash on Android

[tomasMizera]

Moved the call to parent destructor to the last line and added a check if the locationManager is already NULL.
Though it is not clear why it would be NULL

https://app.clickup.com/t/862ke67ea

[PeterPetrik]

This was 20% of recent crashes on android play

Exception java.lang.RuntimeException:
  at android.app.ActivityThread.handleStopService (ActivityThread.java:5293)
  at android.app.ActivityThread.-$$Nest$mhandleStopService
  at android.app.ActivityThread$H.handleMessage (ActivityThread.java:2452)
  at android.os.Handler.dispatchMessage (Handler.java:106)
  at android.os.Looper.loopOnce (Looper.java:226)
  at android.os.Looper.loop (Looper.java:313)
  at android.app.ActivityThread.main (ActivityThread.java:8757)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:571)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1067)
Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'void android.location.LocationManager.removeUpdates(android.location.LocationListener)' on a null object reference
  at uk.co.lutraconsulting.PositionTrackingService.onDestroy (PositionTrackingService.java:88)
  at android.app.ActivityThread.handleStopService (ActivityThread.java:5273)