Fix onDestroy crash (fix #29)

src/org/traccar/client/TraccarService.java

@@ -82,12 +82,18 @@ public class TraccarService extends Service {
     public void onDestroy() {
         StatusActivity.addMessage(getString(R.string.status_service_destroy));
 
+        if (sharedPreferences != null) {
         	sharedPreferences.unregisterOnSharedPreferenceChangeListener(preferenceChangeListener);
+        }
 
+        if (positionProvider != null) {
         	positionProvider.stopUpdates();
+        }
 
+        if (clientController != null) {
         	clientController.stop();
         }
+    }
 
     private PositionProvider.PositionListener positionListener = new PositionProvider.PositionListener() {