Whenever the font in the Context (e.g. through selecting another font
from the fonts list) changes, a signal is emitted to propagate the
change to the active text layer, but if the font is set to NULL in
Context, we don't want that to propagate (because this would change the
current text's font to some fallback font).
In other similar codepaths (e.g. brushes) this is not a problem because
selecting a brush that doesn't exist will prevent painting until an
existing brush is selected.
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