Passing parameter type error when executing command 'topsApp.py --steps'

[zhbfang]

Dear scott:
I am an InSAR rookie, just started to get in touch with Linux,so this problem may be a kind of low-level and it is an honor to ask you questions.If you are willing to help me, I will be very very grateful.
When I installed ISCE according to your instructions"Notes for installing ISCE 2.2.0 (on Ubuntu 18.04 LTS)", I executed the command"topsApp.py --steps",then there is a parameter type error like:

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File "/opt/isce-2.2.0/install/isce/applications/topsApp.py",line 901,in
insar.run()
File "/opt/isce-2.2.0/install/isce/components/iscesys/Component/Application.py",line 129,in run
exitStatus = self._processSteps()
File "/opt/isce-2.2.0/install/isce/components/iscesys/Component/Application.py",line 392,in _processSteps
result = func(*pargs,**kwargs)
File "/opt/isce-2.2.0/install/isce/components/isceobj/TopsProc/Factories.py",line 40
,in call
return self.method(self.other,*args,**kwargs)
File "/opt/isce-2.2.0/install/isce/components/isceobj/TopsProc/runPreprocessor.py",line 35,in runPreprocessor
frame.output = os.path.join(frame.output,'IW{0}'.format(swath))
File "/usr/lib/python3.6/posixpath.py",line 80,in join
a = os.fspath(a)
TypeError:expected str,bytes or os.PathLike object,not NoneType

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