[racket] How to read custom editor-snip% from WXME file

I created an extended editor-snip% class (also using the graph-snip-mixin)
and put an instance of this on a pasteboard% (altered
via graph-pasteboard-mixin) and then saved the pasteboard to a file.

Reading this file back into the pasteboard fails (no visible errors
either). Doing the same with a non-extended editor-snip does work, so the
only change is the extension class.