Cannot Add Elements to AIPerceptionStimuliSource's Register as Source on an Instance in the Level

When attempting to add an element to the Register as Source for Senses on an instance of a blueprint in the level, the + button has no effect. Clicking the button does not add an element.

This works as expected inside the blueprint itself, just not on an instance of the blueprint that is already in the level.

Found in 4.12 binary. Reproduced in 4.11.2 binary and Main CL 2994701

1. Open the editor
2. Create a new Pawn BP
3. Add an AIPerceptionStimuliSource component
4. Add an instance of the Pawn to the level
5. Select the instance and navigate to the details panel
6. Select the AIPerceptionStimuliSource component
7. Click the + to add an element to Register as Source for Senses

Result: No element is added.

Expected: Element would be added successfully.