Interpolate (generate from path) doesn't work on tutorial file (parsing issue)

Tutorials files are optimised, the error message explains the issue
Issue found by @ahjun001 in inbox#2030 (comment 304471031)

Steps to reproduce

1. Open Inkscape
2. Open drawing.svg or the tutorial in Help > Tutorials > Inkscape: Interpolate
3. Select two shapes for interpolating (from the tutorial, use the example shapes, rather than random text)
4. Extensions > Generate from Path > Interpolate

Also affects Generate from Path > Extrude and other stuff probably.

Traceback (most recent call last):
  File "interp.py", line 256, in <module>
    Interp().run()
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/base.py", line 123, in run
    self.save_raw(self.effect())
  File "interp.py", line 71, in effect
    start = elem1.path.to_superpath()
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/elements.py", line 395, in path
    return Path(self.get_path())
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/paths.py", line 1014, in __init__
    for item in (path_d or ()):
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/paths.py", line 1030, in parse_string
    args = list(strargs(numbers))
  File "/home/nal/all/inkscape/share/inkscape/extensions/inkex/utils.py", line 176, in strargs
    return [kind(val) for val in string.replace(',', ' ').split()]
ValueError: invalid literal for float(): 18.266-18.266

Example path: m -47.930545-56.342207 h 87.08808 V 47.690869 h -87.08808 z or (from the tutorials) m255.44 2.0241c10.119 0 18.266 8.1464 18.266 18.266s-8.1464 18.266-18.266 18.266-18.266-8.1464-18.266-18.266 8.1464-18.266 18.266-18.266z

File with those paths for testing: drawing.svg

Inkscape version

1.1-dev (d07754597d, 2020-03-14) using extensions at 1b5cc38a
1.1-dev (1:0.92.0+devel+202003111342+8523277) using extensions from master (from a couple days ago)

Working correctly in Inkscape 0.92.4 (8da668285a, 2020-02-28, custom)