Doc type editor error messages not shown and prevent saving

Reproduction path:

• document type A is the parent of type B
• use an inherited field of type A in type B
• open document type A in the document type editor and try to remove the field that is used in type B
• Result: nothing happens, the field is not removed
• Refresh the browser: now an error message shows up. See screenshot. This should happen right away.
• Also: saving the document type is not possible anymore. There is no way to get out of the error state. Well: log in and log out again, so maybe this is not such a big problem.