Check for null outcomeEventsController when calling its methods

- Use lazy initialization and singleton pattern to access and create outcomeEventsController
- When we call outcomeEventsController's methods, check if it is null, and if so, initialize it
- Follow https://www.infoworld.com/article/2077568/java-tip-67--lazy-instantiation.html

Author: nan-li

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java

@@ -442,6 +442,22 @@ static OSSharedPreferences getSharedPreferences() {
    @Nullable private static OSOutcomeEventsController outcomeEventsController;
    @Nullable private static OSOutcomeEventsFactory outcomeEventsFactory;
    @Nullable private static OSNotificationDataController notificationDataController;
+   private static final Object outcomeEventsControllerSyncLock = new Object() {};
+
+   static OSOutcomeEventsController getOutcomeEventsController() {
+      if (outcomeEventsController == null) {
+         synchronized(outcomeEventsControllerSyncLock) {
+            if (outcomeEventsController == null) {
+               if (outcomeEventsFactory == null) {
+                  OneSignalDbHelper dbHelper = getDBHelperInstance();
+                  outcomeEventsFactory = new OSOutcomeEventsFactory(logger, apiClient, dbHelper, preferences);
+               }
+               outcomeEventsController = new OSOutcomeEventsController(sessionManager, outcomeEventsFactory);
+            }
+         }
+      }
+      return outcomeEventsController;
+   }
 
    @SuppressWarnings("WeakerAccess")
    public static String sdkType = "native";
@@ -834,7 +850,7 @@ synchronized private static void init(Context context) {
       initDone = true;
       OneSignal.Log(LOG_LEVEL.VERBOSE, "OneSignal SDK initialization done.");
 
-      outcomeEventsController.sendSavedOutcomes();
+      getOutcomeEventsController().sendSavedOutcomes();
 
       // Clean up any pending tasks that were queued up before initialization
       taskRemoteController.startPendingTasks();
@@ -871,8 +887,7 @@ private static void setupContextListeners(boolean wasAppContextNull) {
             outcomeEventsFactory = new OSOutcomeEventsFactory(logger, apiClient, dbHelper, preferences);
 
          sessionManager.initSessionFromCache();
-         outcomeEventsController = new OSOutcomeEventsController(sessionManager, outcomeEventsFactory);
-         outcomeEventsController.cleanCachedUniqueOutcomes();
+         getOutcomeEventsController().cleanCachedUniqueOutcomes();
       }
    }
 
@@ -950,7 +965,7 @@ private static void doSessionInit() {
          logger.debug("Starting new session with appEntryState: " + getAppEntryState());
 
          OneSignalStateSynchronizer.setNewSession();
-         outcomeEventsController.cleanOutcomes();
+         getOutcomeEventsController().cleanOutcomes();
          sessionManager.restartSessionIfNeeded(getAppEntryState());
          getInAppMessageController().resetSessionLaunchTime();
          setLastSessionTime(time.getCurrentTimeMillis());