When creating the variables, do not set a value for either. Default Input Action Row is used in Event OnInitialized to keep a reference to the Button’s default setting and Null Input Action is used in the same Event to set the Button’s Triggering Input Action to nothing.
The Button should have a Triggering Input Action set to whatever value you desire and this must be set both in the Button’s Blueprint and any instance of the Button in a Widget Blueprint (i.e., updating the Button’s Blueprint doesn’t seem to update the settings in existing Instances).
The flow is:
-Button is added to a Widget Blueprint and its Triggering Input Action is set to whatever is desired when the Button is Hovered by Mouse or by Keyboard.
-On Runtime, Button is Initialized and its Triggering Input Action is stored in Default Input Action Row and set to Null. It won’t respond to Inputs when it isn’t Hovered.
-Button is Hovered by Mouse or Keyboard and its Triggering Input Action is set to the Default Input Action Row. It will respond to Inputs.
This is how only one Button - the Hovered one - will respond to an Input.