Traceback (most recent call last):
  File "/usr/share/inkscape/extensions/perspective.py", line 127, in <module>
    e.affect()
  File "/usr/share/inkscape/extensions/inkex.py", line 154, in affect
    self.effect()
  File "/usr/share/inkscape/extensions/perspective.py", line 80, in effect
    solmatrix[i][0] = sp[i][0]
UnboundLocalError: local variable 'sp' referenced before assignment