Are you able just Get Game Instance (not custom node but UE4 node) and cast it to ShooterGameInstance? It will still get null? If yes there and you are sure that ShooterGameInstance is selected in Project Setting there might be issue with GameInstances in UE4. Then you should post it on answer hub.
Try to create new game instance class -> assign it to game instance in project setting -> check if you can get reference to it.