`TypeError` in `ExceptionsTable.update`

[hendrikmakait]

https://github.com/dask/distributed/blob/main/distributed/dashboard/components/scheduler.py#L3293 fails if ts.erred_on is None.

Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]: 2022-08-11 13:40:51,814 - bokeh.util.tornado - ERROR - Error thrown from periodic callback:
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]: 2022-08-11 13:40:51,816 - bokeh.util.tornado - ERROR - Traceback (most recent call last):
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/tornado/gen.py", line 526, in callback
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     result_list.append(f.result())
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/server/session.py", line 95, in _needs_document_lock_wrapper
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     result = func(self, *args, **kwargs)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/server/session.py", line 229, in with_document_locked
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     return func(*args, **kwargs)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/document/callbacks.py", line 450, in wrapper
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     return invoke_with_curdoc(doc, invoke)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/document/callbacks.py", line 408, in invoke_with_curdoc
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     return f()
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/document/callbacks.py", line 449, in invoke
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     return f(*args, **kwargs)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/distributed/dashboard/components/__init__.py", line 41, in <lambda>
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     doc.add_periodic_callback(lambda: update(ref), interval)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/core/property/validation.py", line 95, in func
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     return input_function(*args, **kwargs)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/distributed/dashboard/components/__init__.py", line 49, in update
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     comp.update()
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/bokeh/core/property/validation.py", line 95, in func
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     return input_function(*args, **kwargs)
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:   File "/opt/conda/envs/coiled/lib/python3.10/site-packages/distributed/dashboard/components/scheduler.py", line 3293, in update
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]:     new_data["Worker(s)"].append(",\n".join(ts.erred_on))
Aug 11 13:40:51 ip-10-0-9-12 cloud-init[1262]: TypeError: sequence item 0: expected str instance, NoneType found

(https://cloud.coiled.io/dask-engineering/clusters/48659/details)