this should fix makehumancommunity#182 ...

_gitcmd is set to None in library gitutils.py, when it is not found it
stay None.  When you use isfile with a NoneType ... it crashes. Easily fixed by adding
an or statement :)

Author: black-punkduck

makehuman/download_assets_git.py

@@ -88,7 +88,7 @@ def __init__(self):
 
         self.readOverridesFromEnvironment()
 
-        if not os.path.isfile(self._git_command):
+        if self._git_command is None or not os.path.isfile(self._git_command):
             print("\n\n\nCould not find git command\n\n")
             sys.exit(1)