http://localhost/dask/clusters 404 (Not Found) - when running Dask helm chart on GKE

[I'm running Kubernetes on GKE](https://zero-to-jupyterhub.readthedocs.io/en/latest/google/step-zero-gcp.html), following steps 1-6. I've installed Dask on the cluster using helm:
```
helm repo update
helm install stable/dask
```

I believe the chart is from https://github.com/helm/charts/blob/master/stable/dask/Chart.yaml.

The kubernetes service is running the `dask-3.1.0` chart. I confirmed this with
`kubectl get svc -o yaml`.

The kubernetes `dask-jupyter` pod is running the `daskdev/dask-notebook:1.1.5` Docker image. I confirmed this with
`kubectl get pods --all-namespaces -o=jsonpath='{range .items[*]}{"\n"}{.metadata.name}{":\t"}{range .spec.containers[*]}{.image}{", "}{end}{end}' |\
sort`

When running the jupyter notebook, I am seeing a lot of these errors in the Chrome console (Ctrl+Shift+J). They appear every few seconds:
GET http://10.10.0.50/dask/clusters?1555537293299 404 (Not Found)

Replace the ip address with the ip address of the kubernetes node running the jupyterhub notebook.

What I've tried:
- Running the dashboard in mozilla with all browser extensions disabled
- Restarting the cluster
- Running the cluster on multiple wifi networks

My issue seems to be the same issue as https://github.com/dask/dask/issues/4711. But I see that `dask-labextension` is now installed in the [daskdev Docker image](https://github.com/dask/dask-docker/blob/master/notebook/Dockerfile), which is pulled in by the [dask helm chart](https://github.com/helm/charts/blob/master/stable/dask/values.yaml). So it's unclear to me what to investigate next.