Observing isCompatible bug when using bezier tool

[Guest]

Hello again! My weekend robofont training (self imposed) continues, and I may have discovered a bug.

I am attempting a live update on the compatibility of two glyphs from different fonts.
The idea at its most basic could manifest as a red mark drawn to the glyph window canvas whenever the glyph becomes incompatible whilst you are drawing/editing the glyph.

I am using isCompatible() and a glyph.changed observer

It reports when I make a change to the glyph, calling a function that for now simply prints the result of isCompatible.

It works fine if I remove a contour, and it works fine if I add a point to an existing contour using knife, but it seems to crash if I use the bezier tool to start drawing a new contour.

I have a gist here, please try to replicate the error.
https://gist.github.com/MBurvill/fb6772f9e8bc49e583f16e6dc650b7d8#file-isCompatibleObserverBugTest-py

FYI I found the useage of glyph.changed in one of Frederiks examples (stencil preview), but usage of this observer appears to be undocumented, I would expect to be able to find it at http://doc.robofont.com/api/custom-observers/

Thanks!