Safari freezes after sharing a webpage to Agenda Notes

What I did:
Clicked the Share button in Safari and choose AgendaNotes. Chose a project, typed in a remark and saved the link.

What happened:
When I switched back to Safari, I found the whole app to be unresponsive. It didn’t crash, I could still see the webpage I’d shared before, but it no longer responded to mouse events. BTW, this is not the first time this happened, it has happend in the past week several times.

What I expected:
I expected to be able to go on using Safari as usual. Instead, I had to Force stop via Finder’s Apple Menu (force-quitting via the Dock icon did not work).

Things that might be helpful to know (Agenda version, OS and model, etc):
MacOS Catalina 10.15.7 (19H524). Agendanotities (Dutch version) Versie 12.0.2 (181) .

That’s very odd, we haven’t heard of this before. Does this still happen after a reboot of the Mac?