With the latest version this error occurs on a regular basis just by browsing through the main grid. There is no definite pattern like a certain number of clicks in the grid but it's repeatable without problems.
Essentially, it looks like this: all docked windows are loosing their tab controls (it happened also to the tabbed details view / edit view window, I just didn't add a screenshot for this):
So, since I have three docked windows with tabs (left side: tree / mini stat + Front / Back cover; right side Details / Quick Edit) the error occurs three times per session. Once I've lost all three tab controls it seems "OK" for XMM ... but not for me.
OK, some more details / an update to my first post: a little while after all docked windows already lost their tabs I've got yet another exception (missing object reference).