Check for reactContext before emitting appLaunched event

If ReactContext is null, the event is emitted when onReactContextInitialized is invoked. Closes #3392

lib/android/app/src/main/java/com/reactnativenavigation/react/NavigationReactInitializer.java

 		if (shouldCreateContext()) {
 			reactInstanceManager.createReactContextInBackground();
 		} else if (waitingForAppLaunchEvent) {
-			emitAppLaunched(reactInstanceManager.getCurrentReactContext());
+            if (reactInstanceManager.getCurrentReactContext() != null) {
+			    emitAppLaunched(reactInstanceManager.getCurrentReactContext());
+            }
 		}
 	}