Attach (recommended) or Link to PDF file here:
PDF from #4106, https://mail.gnome.org/archives/evince-list/2013-November/pdfNC0hkVW67a.pdf.

Steps to reproduce the problem:
1. Open the PDF

What is the expected behavior?
![immagine](https://user-images.githubusercontent.com/1616846/121865567-67ca4200-ccfe-11eb-8673-1d6e863fc8d5.png)

What went wrong?
The PDF doesn't load, `TypeError: can't access property Symbol(), match is null` error is logged in the console.
yield* getContainedChildren(this);
  }
  [$getSubformParent]() {
    let parent = this[$getParent]();
    while (!(parent instanceof Subform)) {
      parent = parent[$getParent]();
    }
    return parent;
  }
}
class SubjectDN extends ContentObject {