Currently, a certain sequence of undo/redo actions can cause a Trigger's Properties Panel to go completely blank until the document is closed and re-opened.

See attached video.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Select the RootGrid, and give it a Background colour of your choice.

3) With the RootGrid still selected, give it an OpacityMask of type 'LinearGradient'

4) Navigate to the Animation/Storyboards Panel, and create a new Storyboard named "MyStoryboard"

5) With the "MyStoryboard" Timeline open, at the 0s mark, select the RootGrid, hover over its "OpacityMask" 3-Dot Menu and select "Create Keyframe".

6) Navigate to the Interactivity/Triggers Panel, and create a new EventTrigger named "MyTrigger" applied the RootGrid (which should now be named "Grid0").

7) Set the EventTrigger's to trigger a "PlayStoryboardAction", and for it to trigger the "MyStoryboard" Storyboard created in Step 4).

8) Return to the Animation/Storyboards Panel, and enter the "MyStoryboard" Timeline once again.

9) Move the Playhead to the 1s mark, select the RootGrid, hover over its "OpacityMask" 3-Dot Menu and select "Create Keyframe".
(There does not need to be a change in value necesarily).

10) Return to the Interactivity/Triggers Panel and STOP without selecting anything in the UI after clicking on the 'Interactivity' Tab.
(This is important. If you select the EventTrigger, the bug cannot be reproduced).

11) Using the Application Hamburger Menu's "Edit-->Undo" function, perform 'undo' actions until the Trigger created in Step 6) is no longer visible in the Interactivity/Triggers Panel.

12) Using the Application Hamburger Menu's "Edit-->Redo" function, perform 'redo' actions until the Trigger created in Step 6) is visible once again. At this point, you may do a couple further 'redo' actions just for good measure.

13) Click on the Trigger in the Interactivity/Triggers Panel.

Observe how although it can be selected, its Properties no longer display.

14) Click all around the UI, on various Elements, Various Tabs, etc...
Observe how all other functions in the UI display a Properties Panel.

15) Return to the Interactivity/Triggers Panel, and click on the EventTrigger once again.

Observe how the Trigger still fails to display its Properties Panel.

16) Save the open document, re-open it, Return to the Interactivity/Triggers Panel, and click on the EventTrigger once again.

Observe how the Trigger's Properties now appear.

Expected Results: The Triggers Properties never cease from being displayed as a result of the undo/redo stack.