I have encountered this too on two sites when creating a Block Element and then using the Block Element dropdown.
Both sites are updated to WordPress 6.2. All plugins are up to date and GP is also up to date.
The error message only occurs when choosing the Hook Element Type and then the Hook Name.
If you ignore the error message and save, you then lose all the active fields under the Element tab. But if you then leave the page and open it again from the Admin Element list you see that your work has been saved and the hook works.
The error occurs again if you alter the Hook Name