I didn’t understand why my external application icon in the top left of the toolbar for rich text was greyed out. I did a Get Info on a similar file in the Finder and the “open all documents like this” was set correctly (Bean.app).
Confused. Wasting time.
What about this message when a user clicks on a greyed out external application Icon:
“You need to save your document before you can view it in an external application.”