crash on invalid instance input

[typemytype]

[typemytype]

Traceback (most recent call last):
  File "/Users/frederik/Library/Application Support/RoboFont/plugins/DesignspaceEditor2.roboFontExt/lib/designspaceEditor/tools.py", line 214, in stringForObjectValue_
    return formatNumber(obj)
           ^^^^^^^^^^^^^^^^^
  File "/Users/frederik/Library/Application Support/RoboFont/plugins/DesignspaceEditor2.roboFontExt/lib/designspaceEditor/tools.py", line 202, in formatNumber
    value = float(value)
            ^^^^^^^^^^^^
ValueError: could not convert string to float: 'a'

[LettError]

I can't repeat.