Looks like I may have found a bug in the designer.
I added an image view and tried to add some members to it and was able to do so. Next I tried to do the same thing to the custom views and I was able to do it without any errors.
The error originally happened after I copied and pasted a custom view from another app so it seems the coping and pasting caused the error to surface.
An activity module was open at the time but the next time it happens, I will try closing the designer and clicking through the other modules and returning to the activity module or closing the app and re-opening it and try again.