Profile tab broken on worker dashboard

**What happened**:
Nothing shows up on page. From console logs:


**Minimal Complete Verifiable Example**:

```python
from distributed import Client, LocalCluster

cluster = LocalCluster(worker_dashboard_address=":0")
client = Client(cluster)
client
```

Go to `http://127.0.0.1:8787/proxy//127.0.0.1/profile`

I discovered this on bokeh 2.2.0rc1, but it was also the case on bokeh 2.1.1, so not a recent regression.