Critical patch released for 0.9.23

A patch release was just posted. You may see a notice that an update is available for your version - if you see this, re-download and re-install it.

This patch fixes an issue in the sharing system that could cause a crash if you loaded a file, edited nodes on a shape, then undid all your changes, and re-saved.  Since it could also crash when auto-saving, it is likely the cause of a number of seemingly random crashes, so we're very happy to have this one fixed.

This is the only change for this patch.