Make contextual balloon panel view skip empty targets

[jodator]

I've been struggling with some errors on removing a panel from contextual balloon.

The issue is that when a view is removed from the balloon, the balloon uses old position to display new panel in the same placement as removed one. This works well if position target is static. When there's callback used the callback may operate on removed data - ie something that is not available in the view and doesn't have anything to point. It would be nice to return null or undefined from target() callback so the balloon plugin would use new position to display such element. Right now it uses the last one: https://github.com/ckeditor/ckeditor5-ui/blob/b34e9d553d70997551310cee531f95b69f88de5b/src/panel/balloon/contextualballoon.js#L425

while the view is removed from the stack later:
https://github.com/ckeditor/ckeditor5-ui/blob/b34e9d553d70997551310cee531f95b69f88de5b/src/panel/balloon/contextualballoon.js#L236-L247

Also the problem was that both panels were removed but the logic for updating balloon position was called for both removed panels.

[CKEditorBot]

There has been no activity on this issue for the past year. We've marked it as stale and will close it in 30 days. We understand it may be relevant, so if you're interested in the solution, leave a comment or reaction under this issue.

[CKEditorBot]

We've closed your issue due to inactivity over the last year. We understand that the issue may still be relevant. If so, feel free to open a new one (and link this issue to it).