Steps to reproduce:
- Visit a diff which has been deleted, eg. https://de.wikipedia.beta.wmflabs.org/w/index.php?title=Benutzer:Schnark&diff=2074&oldid=1945&visualdiff=1
- Switch to visual diff.
- If you have the right to view the deleted diff, do so.
- Switch to visual diff again.
The visual diff fails to load in both cases, and just shows the loading bar.
When you are not allowed to view the diff, the switch widget should either don't exist at all, be disabled, or don't do anything except telling you that you can't view the diff.
I don't know whether it is possible to show visual diffs for deleted revisions you have the right to view. If it is possible, then the diff should be shown, if RESTbase isn't able to do so, the switcher should be disabled for such diffs.