fix: [SDK-4722] guard getNotifications() in onDetachedFromEngine (#1149) (#1150)

Author: fadi-george

.github/pull_request_template.md

@@ -228,6 +228,12 @@ public void onNotificationPermissionChange(boolean permission) {
     }
 
     void onDetachedFromEngine() {
+        // The Flutter engine can be torn down before OneSignal.initialize() has been
+        // called from Dart (cold start, fast finish, etc.). Calling getNotifications()
+        // in that state throws IllegalStateException from the native SDK. See #1149.
+        if (!OneSignal.isInitialized()) {
+            return;
+        }
         // Unsubscribe so clicks while the engine is dead get queued by the native SDK
         // instead of dispatched on a detached channel.
         OneSignal.getNotifications().removeClickListener(this);