fix: Prevent null eventSink crash in onSharedPreferenceChanged (#449)

- Added null check before calling `eventSink.success(result)`
- Fixes intermittent crashes when reloading the app
- Resolves #449

Author: DrumRobot

packages/activity_recognition_flutter/android/src/main/java/dk/cachet/activity_recognition_flutter/ActivityRecognitionFlutterPlugin.java

@@ -168,7 +168,7 @@ public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, Strin
         // Log.d("onSharedPreferenceChange", result);
         if (key!= null && key.equals(DETECTED_ACTIVITY)) {
             // Log.d(TAG, "Detected activity: " + result);
-            eventSink.success(result);
+            if (eventSink != null) eventSink.success(result);
         }
     }
   }