Traceback (most recent call last):
  File "lib/doodleDelegate.pyc", line 97, in sendEvent_
  File "/Applications/RoboFontBeta3.2b.app/Contents/Resources/lib/python3.6/vanilla/vanillaWindows.py", line 496, in windowDidBecomeMain_
    self._alertBindings("became main")
  File "/Applications/RoboFontBeta3.2b.app/Contents/Resources/lib/python3.6/vanilla/vanillaWindows.py", line 467, in _alertBindings
    return callback(self)
  File "skateboard.py", line 2992, in mainWindow_becameMain
  File "skateboard.py", line 2762, in document_updateCurrentDesignSpace
  File "skateboard.py", line 2877, in document_checkFonts
  File "genericpath.pyc", line 19, in exists
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

I've also received this similar error:

Skateboard: Failed to open the document /Users/stephennixon/type-repos/route-project/route.designspace
Traceback (most recent call last):
  File "skateboard.py", line 4574, in mainWindow_openDocumentWithPath
  File "skateboard.py", line 2877, in document_checkFonts
  File "genericpath.pyc", line 19, in exists
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

That's all in the Output Window. If I try to click the "Debugs" button of Skateboard, nothing happens, but I get this in the Output Window:

Skateboard: Ironically, can't open the debug window because of a problem. Traceback (most recent call last):
  File "skateboard.py", line 2961, in debug_makeObjectBrowserCallback
  File "skateboard.py", line 2961, in <listcomp>
AttributeError: 'NoneType' object has no attribute 'path'

I like the humor in the last error message, at least. :D

It doesn't really affect things if I restart RoboFont, and it doesn't have any affect if I edit the designspace file.

If there's anything I should try, I'd appreciate any pointers. (And actually, I'll take a couple minutes to restart my computer right now, then report back if that changes anything...).

Update: I've restarted my computer. No luck.

Extra Context:

macOS Mojave v 10.14.2

RoboFont Version 3.2b (built 1809041630)

Note: I have recently been fixing some issues with how I set up my local python3 and pip, so there's a possibility that is part of this problem ... but I'm not sure whether that is interacting with RoboFont/Skateboard. I did also have an issue calling defcon from glyphsLib earlier in the day, so there may be bigger problems afoot. However, I have also been fine in installing and working with several things, so my python isn't completely hosed. I just didn't want to leave out that potential context in case it could be relevant, as my issue might be some kind of edge case.

I mistakenly took the error message to mean that the .designspace path couldn't be resolved, but your comment made me open another designspace (IBM Plex Sans) to check how things were looking. Sure enough, that worked perfectly.

I compared the syntax of the two designspaces ... and sure enough, I had messed mine up: In the <instance> element, I had a file attribute – rather than the necessary filename attribute.

I had straight-up misread the spec on GitHub (LettError/designSpaceDocument).

A small question this leads me to ... is there any good "boilerplate" designspace doc I could grab to start the next project from? I realize the challenge there might be including everything that different people wanted to include, but even so, if there were a simple example complete with an axis, two sources, and three instances, it might be a nice starter to copy-paste from, and help people avoid future syntax issues.

Of course, I could just make this as a GitHub Gist, too, if you don't know of something like that off-hand.

Personally I try to avoid writing the xml by hand (situation, preference and external factors permitting of course) and prefer to write small scripts using designspacelib or ufoprocessor. While the script might be just as verbose as the resulting xml, I find a script to be more explicit. Here's a page on scripting designspace files