The problem is that too many X buttons are being created when the file tabs are auto-restored. I *think* you need to have exactly 2 file tabs in one tab group, and that it only happens when first starting the editor, restoring those 2 files. (It was really bad on the Mac in Beta 1, and hard to reproduce on Windows)
Anyway, this will be fixed in the final release. We're going to be hand-drawing all those close buttons anyway rather than having it be a control.