Panic screen sometimes does not appear when it should on the extension's SWF player page

[n0samu]

Describe the bug

On the the Ruffle extension's internal player page, some error conditions do not make the panic screen appear, even though they should. Instead, either the loading spinner animation plays forever, or a blank screen appears, either of which is very confusing for users.

• If an invalid SWF is loaded via the "Select File" button, Ruffle encounters an error while trying to generate the panic message, and so a blank screen is shown instead of the panic screen:
  Uncaught (in promise) TypeError: this.swfUrl is undefined
• If a non-SWF URL is loaded via the URL box, the loading spinner animation plays forever and the panic screen does not appear. That's despite this error appearing in the console:
  ERROR web/src/navigator.rs:377 Asynchronous error occurred: Invalid SWF: Couldn't read SWF: Invalid data: Invalid SWF

Expected behavior

The panic screen should appear.

Content Location

Example of a non-SWF URL: https://www.miniplay.com/game/renegade-racing

Affected platform

Browser's extension

Operating system

Windows 10

Browser

No response

Additional information

No response