The L"default" font doesn't actually exist, so instead of complaining about missing "fonts/.fnt" it complains about missing "fonts/default.fnt" if it was ever to be the case.
The approach of the GUI is that it reads data from the XML file and complains if one didn't provide a mandatory value, so it seems out of place to do it differently for the fonts.
If one wanted a default font, it should be set alongside the other font values in the constructor instead.

The only cases in which this code was actually triggered is when no font was rendered.
Precisely in the CDropdown constructor called from Xeromyces_ReadObject there are some SetSetting calls that happen to send the settings-changed message which resulted in SetupText calls,
and then those calls read from the default constructed setting values. But then Xeromyces_ReadObject finishes and the font is set.

Consider whether the argument in the summary is approvable. One can add an ENSURE !font.empty and if empty LOGERROR to see in which cases it happens.
Notice that we don't have a default.fnt, so it won't change anything for 0ad nor mods unless the mod would happen to add a default.fnt and happen to make use of this featurebug that only C++ developers noticed.