libgimp: resource button for font throws CRITICAL for null initial choice
Symptom is this in the console:
(script-fu-interpreter:413): LibGimp-CRITICAL **: 13:53:32.714: gimp_resource_select_new: assertion 'resource != NULL' failed
and a chooser widget fails to appear, when you click the button.
The test case is plugin Test>Sphere v3. The widgets for the other resources (brush, etc.) don't show symptoms.
A ResourceChooser button should default the shown choice to the resource in context when a NULL initial resource is passed. It is a hard requirement that the context always has choices for resources, a user can't delete all resources of a kind.
Currently, a plugin can't declare a default, so NULL is always passed, at least on the first run of a plugin. Related to #10950.
The fix is known, the relevant code for gimp_font_chooser_new differs from gimp_palette_chooser_new, but should not differ.