used outside of main canvas
E.g. we shouldn't show scale based visibility text format changes
or the editable toggle in models used for snapping dialog, project
properties, etc.
These properties are only associated with the main map canvas.
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
