AttributeError: 'NoneType' object has no attribute 'thread'

After running a jupyter notebook using `jupyter nbconvert --to notebook --execute --ExecutePreprocessor.timeout=15000 --inplace`, I get the following error:

```
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
File "/usr/lib/python3.5/logging/__init__.py", line 1881, in shutdown
h.flush()
File "/home/kbuilder/.local/lib/python3.5/site-packages/absl/logging/__init__.py", line 882, in flush
self._current_handler.flush()
File "/home/kbuilder/.local/lib/python3.5/site-packages/absl/logging/__init__.py", line 776, in flush
self.stream.flush()
File "/tmpfs/src/tf_docs_env/lib/python3.5/site-packages/ipykernel/iostream.py", line 341, in flush
if self.pub_thread.thread.is_alive():
AttributeError: 'NoneType' object has no attribute 'thread'
```

This is the line where it fails: https://github.com/ipython/ipykernel/blob/master/ipykernel/iostream.py#L341

What can I do to fix it?