Widget in Element desktop times out

### Steps to reproduce

* Receive incoming video call in Element Desktop
* Click "Join"
* Observe spinner

### Outcome

#### What did you expect?

I should be joined to the video call. Or at the very least, an error telling me something went wrong.

#### What happened instead?

Spinner of doom.

In the logs:

```
Widget specified waitForIframeLoad=false but timed out waiting for contentLoaded event!
```

### Operating system

Debian stable

### Browser information

N/A: in Element Desktop Nightly

### URL for webapp

_No response_

### Will you send logs?

Yes