activity_recognition_flutter 4.0.5+1

### Device / Emulator and OS
Android - several models Android 9 - 11

### Describe the bug
Quitting and reloading an application with this plugin crashes periodically on load. Some devices experience this more often than others - maybe only loading successfully once every 4-5 attempts.

Logs show:

```
java.lang.NullPointerException: Attempt to invoke interface method 'void io.flutter.plugin.common.EventChannel$EventSink.success(java.lang.Object)' on a null object reference
at dk.cachet.activity_recognition_flutter.ActivityRecognitionFlutterPlugin.onSharedPreferenceChanged(ActivityRecognitionFlutterPlugin.java:167)
at android.app.SharedPreferencesImpl$EditorImpl.notifyListeners(SharedPreferencesImpl.java:607)
at android.app.SharedPreferencesImpl$EditorImpl.lambda$notifyListeners$0(SharedPreferencesImpl.java:613)
at android.app.-$$Lambda$SharedPreferencesImpl$EditorImpl$3CAjkhzA131V3V-sLfP2uy0FWZ0.run(Unknown Source:4)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7156)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:975)
```
Looking into the code it is eventSink that is null.

Would expect it to load without crashing. Note debugging the app connected to the computer loads fine every time, its only when quitting the app and launching from the device itself that this issue occurs. Happens in both release and debug builds installed via apk if that makes any difference.