Crash would happen in e.g. curve selection operators.
The BSurfaces panel was visible in the active tool settings in the
Properties Editor (because of a 'feature' in bsurfaces that would allow
for a custom tab name -- that defaulted to "Tool").

Solution is to not have this in the "Tool" category, but instead have
this in the Edit category (as advertised in the "location" field as
well), so we only launch stuff from a panel in the sidebar.