Cannot save to sif file. (Synfig porting issue)

Steps to reproduce:

• Open Inkscape.
• Draw vector drawing.
• Click to File -> Save As...
• Save to ***.sif

What happened?

The message appeared:

Inkscape has received additional data from the script executed. The script did not return an error, but this may indicate the results will not be as expected.

Traceback (most recent call last):
  File "synfig_output.py", line 1344, in <module>
    e.affect(output=False)
  File "inkex.py", line 283, in affect
    self.effect()
  File "synfig_output.py", line 1053, in effect
    SynfigPrep.effect(self)
  File "synfig_prepare.py", line 480, in effect
    self.document = a.run_document()
  File "synfig_prepare.py", line 153, in run_document
    self.run_file(svgfile)
  File "synfig_prepare.py", line 133, in run_file
    p = Popen('inkscape "%s" %s' % (filename, cmd), shell=True, stdout=PIPE, stderr=PIPE)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xbe in position 19: ordinal not in range(128)

Inkscape Version and Operating System:

• Inkscape Version: 0.92.4
• Operating System: Windows 10