Error: Haven't Defined an "App ID" for this Page

I periodically get an error telling me: “You haven’t defined an ‘App ID’ for this page.”

But, I have defined it. Multiple times. Hmmm
*EDIT: (added this later for clarity) EVEN THOUGH I HAVE DEFINED IT, WHEN I GO UP TO “APP” THE FIELD IS BLANK AS THOUGH I HAD NOT, FOR SOME REASON IT’S CLEARING IT OUT.

It’s on a route page… if that makes a difference.

I got this just now when trying to add a Boostrap Table Generator

It happens at other times too when adding other components, but I don’t remember which.
Anyone know the cause of such issues?