Strange relation editor entries in window menu which can lead to a stall

[skyper]

What steps will reproduce the problem?

1. Have some mysterious relation editor entry at the bottom of the window menu
2. Click on one
3. In the now open relation editor click on cancel
4. In the following unsaved changes dialog click on anything

What is the expected result?

Some reaction

What happens instead?

No reaction. I cannot close the window.
Only creating more identical windows is possible and the JOSM session is unusable.

Please provide any additional information below. Attach a screenshot if possible.

I had play around with "create multipolygon" in this session and used redo/undo in different layers with possible open relation editors. I probably did even delete layers with open relation editors with unsaved changes. The relation looked like one of this relations where the layer was already deleted.

For the chance that it can help I'll attach a dump​.

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2020-11-18 20:16:58 +0100 (Wed, 18 Nov 2020)
Revision:17323
Build-Date:2020-11-19 02:30:51
URL:https://josm.openstreetmap.de/svn/trunk

[skyper]

thread dump