Hershey Font Extension throws error with line-height:normal

Summary:

In Inkscape 1.1 RC applying the Hershey text extension results in an error

Steps to reproduce:

• open wilma.svg in Inkscape
• select the text, which has line-height:normal
• Extensions->Text->Hershey Text
• Choose any font and apply

What happened?

Throws an error:

Traceback (most recent call last):
  File "hershey.py", line 1939, in <module>
    Hershey().run()
  File "C:\Program Files\Inkscape\share\inkscape\extensions\inkex\base.py", line 131, in run
    self.save_raw(self.effect())
  File "hershey.py", line 1919, in effect
    self.recursively_traverse_svg([self.svg.selected[id_ref]], transform)
  File "hershey.py", line 1413, in recursively_traverse_svg
    line_spacing = self.units_to_userunits(line_spacing_temp)
  File "hershey.py", line 311, in units_to_userunits
    value, _ = units.parse_unit(input_string)
TypeError: cannot unpack non-iterable NoneType object

What should have happened?

The text should have been converted to a svg path font.

Version Info:

• Inkscape Version: Inkscape 1.1-rc (52f87abb86, 2021-05-02)
• Operating System: Windows 10
• Operating System version: 10.0.19042