Follow up with issue: https://github.com/mozilla/pdf.js/issues/14146
Merged pull req: https://github.com/mozilla/pdf.js/pull/14189

Attach (recommended) or Link to PDF file here:
https://bug1708641.bmoattachments.org/attachment.cgi?id=9219459

- This popup appears because JS is enabled;
- And because the pdf contains some unsupported features (e.g. XFA);

Configuration:
- Firefox Nightly & Chrome
- macOS Big Sur 11.6

Steps to reproduce the problem:
1. Run gulp server
2. Open attached pdf

What is the expected behavior? 
No popup on open

What went wrong?
![error](https://user-images.githubusercontent.com/76935202/139354112-707887a2-a6a1-43a9-a92e-45d57c48206a.jpg)



throw new Error("app.fromPDFConverters is read-only");
  }
  get fs() {
    if (this._fs === null) {
      this._fs = new Proxy(
        new FullScreen({ send: this._send }),
        this._proxyHandler
      );
    }
    return this._fs;
  }
  set fs(_) {
    throw new Error("app.fs is read-only");
  }
  get language() {
    return this._language;
  }
  set language(_) {
    throw new Error("app.language is read-only");
  }
  get media() {
    return undefined;
  }
  set media(_) {
    throw new Error("app.media is read-only");
  }
  get monitors() {
    return [];
  }
  set monitors(_) {