Silence error in GraphSheet sidebar

[midichef]

Closes #2601 and #2629.

The error in #2601 comes because a sheet needs to have either default_sidebar or guide. Most sheets have at least a default empty guide because they inherit one from TableSheet. But there are three other subclasses of BaseSheet that are separate from TableSheet: Plotter, TextCanvas, and FormCanvas.

GraphSheet comes from Plotter: Plotter -> Canvas -> InvertedCanvas -> GraphSheet. So I fixed the problem by adding an empty guide to Canvas, to head off the same issue in the several sheet types that are subclasses of Canvas, like GeoJSONMap.