This looks like a really rare crash.
I've looked at the code for a while, and my only explanation is that you've switched really fast from moving pieces to dragging a frame, or the other way around. But I can't reproduce this.
I've made the mouse dragging more robust and hope that this won't fail again. This hope-fix goes in the next version. But I'm not sure, therefore I've added even more asserts.
The editor should have saved your level as levels/editor-emergency-save.txt. You can delete this file when you don't need it anymore.
I'm happy, but also confused, because your crash shows an assertion failure inside an invariant. Neither assertions nor invariants should exist in my release build. Did you compile a debugging version yourself? Or was this one of my test versions that I gave you, I think I built that with debugging flags?
-- Simon