Opening Dialog from Ribbon Custom Action

I've added HostWebDialog="TRUE" (+Width & Height) to my ribbon custom action, and for the life of me I just can't get it to open in a dialog...

I think I have the same issue as Laxmikant in his initial post here:

https://social.msdn.microsoft.com/Forums/sharepoint/en-US/7006270d-90e4-4351-9360-e5ac89954719/modal-dialog-for-custom-action?forum=appsforsharepoint

I do get warnings that the "HostWeb..." attributes are not declared.

I'm thinking maybe it is not working because my custom action is scoped to a list instance in the app web, where the attributes seem to be relevant for the host web.

Does anyone have an idea why it may not be working for me?