[BeckyEbook]

If user never changed font in Calibre > Preferences > Look & Feel then in gui.json not exist "font" section.

Spoiler:

Traceback (most recent call last):
  File "calibre_plugins.calibrespy.ui", line 105, in run_calibrespy_from_gui
  File "calibre_plugins.calibrespy.calibrespy_dialog", line 128, in __init__
  File "calibre_plugins.calibrespy.calibrespy_dialog", line 1005, in build_fonts
TypeError: 'NoneType' object has no attribute '__getitem__'

This code works (tested in Windows only):

Spoiler:

        if calibre_font is not None:
            family = calibre_font[0]
            size = calibre_font[1]
        else:
            if iswindows:
                family = 'Segoe UI'
                size = 9
            else:
                family = 'Sans Serif'
                size = 10

Additional question on the screenshot.