There is an action called "Game Object is Null". You can set it to everyframe, so it will keep checking. It checks to see if the game object variable has anything in it yet.
Once the variable is populated, it can send an event. Just set the "Is Not Null" event, and leave the "Is Null" to nothing. Or maybe put another state with a wait function and check back after X amount of time (if you want).