AOI Shapefile is not optional

The preprocessor treats the AOI as required, at least at the point where it parses the args. So either, 
A) the root UI should either require the AOI,
B) the preprocessor should tolerate the missing value

https://community.naturalcapitalproject.org/t/root-error-typeerror-expected-str-bytes-or-os-pathlike-object-not-nonetype/4177/3