Generate from Path->Interpolate error if fill or stroke unset

Steps to reproduce

• create two paths
• have an unset fill and/or stroke for either/both paths
• run Extensions->Generate from Path->Interpolate , with Interpolate Style checked
• get attached warning:

Traceback (most recent call last):
  File "interp.py", line 283, in <module>
    Interp().run()
  File "C:\APP\Temp\inkscape_devel\share\inkscape\extensions\inkex\base.py", line 141, in run
    self.save_raw(self.effect())
  File "interp.py", line 101, in effect
    if isgradient(sst['stroke']) and isgradient(est['stroke']):
  File "interp.py", line 99, in <lambda>
    isgradient = lambda x: x.startswith('url(#')
AttributeError: 'NoneType' object has no attribute 'startswith'

Occurs in extensions at master. The current extensions for 1.1-dev still suffer from the issue below

Version info in comments, including Inkscape 1.1-dev (dbb3dfc0c3, 2020-08-23, custom) Linux Mint 20

Old bug (fixed in extensions at master)

- Windows 10, Inkscape 1.1-dev (0741f45, 2020-08-20)

• run Extensions->Generate from Path->Interpolate
• get attached warning:
• after this the extension appears to execute
• however, if the two objects have different color, then it appears that the color is not being interpolated

..................................................

interp.py:115: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if strokestyle is 'color':
interp.py:125: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if fillstyle is 'color':