404 GET /dask/dashboard/... in console

When shutting down a LocalCluster I receive 404 GET /dask/dashboard/.../individual-plots.json? messages in the console window E.g.

```
[W 09:30:37.716 LabApp] 404 GET /dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071/individual-plots.json?1609925437712 (::1) 1.440000ms referer=http://localhost:8888/lab
[W 09:30:41.721 LabApp] 404 GET /dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071/individual-plots.json?1609925441717 (::1) 1.450000ms referer=http://localhost:8888/lab
[W 09:30:45.728 LabApp] 404 GET /dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071/individual-plots.json?1609925445724 (::1) 1.340000ms referer=http://localhost:8888/lab
[W 09:30:49.733 LabApp] 404 GET /dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071/individual-plots.json?1609925449729 (::1) 1.290000ms referer=http://localhost:8888/lab
[W 09:30:53.740 LabApp] 404 GET /dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071/individual-plots.json?1609925453736 (::1) 1.330000ms referer=http://localhost:8888/lab
[W 09:30:57.746 LabApp] 404 GET /dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071/individual-plots.json?1609925457742 (::1) 1.330000ms referer=http://localhost:8888/lab
```

If I restart jupyter lab, these messages continue appear even when I am not working with dask.

They stop if I start a new cluster but they reappear if the cluster is shutdown again.

It's not a big issue but more of an annoyance as I may miss important messages in the console as the warning repeats every two seconds.

I suspect it is related to how the dask-dashboard-launcher entry in the lab-xxx.jupyterlab-workspace file (located in .jupyter/lab/workspaces) is used. E.g.

```
"dask-dashboard-launcher": {
"url": "http://localhost:8888/dask/dashboard/e928b78c-1bb6-4a99-92f1-7c32e4f05071",
"cluster": "00566038-4b2d-4b95-b482-455f07eaa996"
}
```

**Environment**:

- Dask version: 2.11.0
- Python version: 3.8
- Operating System: MacOS
- Install method: Conda

Let me know if you require further information