Hi there,
I know what the issue is and will be patching it now.
Turns out this bug was already fixed on a previous version of the plugin but on the newest version I joined every version together to make it easier for me to update them, when I did the fix I added got lost somewhere (probably because I based myself on 5.4 which didn’t have the fix at the time).
The reason this is triggering is because you closed the “Asset Browser” window on the editor, the plugin is kinda assuming that is open which is why it’s crashing (the fix added more defensive programming).
The way to fix this is pretty easy, open your “AnimationSource” (not the AnimSequence), and then click “Window->Asset Browser” which will open that tab again. After that you are free to open the AnimSequences. I’ll add the fix for this soon, but in the meantime keep this opened.