I think you might want to look at the new update that went live yesterday, where I added support for just the sort of thing you are after. I added a new ability called BP_AbilityBase_Idle which is active when no other abilities are active, including when no units are selected. By default it is set up for selecting units, which I think is what you’re after. As you have found most input is handled through the player controller, which is generally best practice in UE4.