Comment 3 for bug 1760493

So another addition (and workaround):

After applying an extension (while having selection sets in the document) -
1) Don't move anything etc. since this will make Inkscape crash.
2) But: Save the file (preferably under a new name).
3) In the menu choose: File>Revert.

After that it is possible to continue working with the document as expected.
(This also suggests that there is probably something wrong with reading in the document after applying the extension modifications in the extensions pipeline.)