automodapi fails on certain modules only

I have a package with Sphinx documentation at https://sdrast.github.io/Data_Reduction/. `automodapi` fails on certain modules in the package with the error:
```
Exception occurred:
File "/usr/local/anaconda3/lib/python3.7/site-packages/sphinx_automodapi/utils.py", line 99, in find_mod_objs
fqnames.append(obj.__module__ + '.' + obj.__name__)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
The full traceback has been saved in /tmp/sphinx-err-7q488zfz.log, if you want to report the issue to the developers.
```
This happens after compiling the module because if there is an error in the module it reports that first. For now I comment out those modules in index.rst (as you'll see if you look at the web page linked above).
[sphinx-err-7q488zfz.log](https://github.com/astropy/sphinx-automodapi/files/4773771/sphinx-err-7q488zfz.log)

Reproduce the failure using the linked Data_Reduction documentation and inspect the traceback in sphinx-err-7q488zfz.log; compare the affected modules referenced by index.rst. Done means automodapi processes those modules without the reported TypeError and the documentation builds without commenting them out.

Written by the indexing model from the issue text.