Attach (recommended) or Link to PDF file here: [ACORD 0211 2016-09 Acroform BLANK.pdf](https://github.com/mozilla/pdf.js/files/6353664/ACORD.0211.2016-09.Acroform.BLANK.pdf)

Configuration:
- Web browser and its version: Google Chrome Version 90.0.4430.72 (Official Build) (x86_64)
- Operating system and its version: Windows 10 Pro 20H2
- PDF.js version: 2.7.570
- Is a browser extension: No

Steps to reproduce the problem:
1. Load attached form into PDFViewer
2. Call saveDocument method on loaded document proxy

What is the expected behavior: should not crash

What went wrong? (add screenshot)
It crashes in writer.js. Line 273 as of 21 Apr 2021.
![image](https://user-images.githubusercontent.com/10527821/115615989-4d747900-a2f8-11eb-8e6a-fa3070d813a7.png)

Line that crashes: 
![image](https://user-images.githubusercontent.com/10527821/115616056-6846ed80-a2f8-11eb-8df1-b4633d3e5e8e.png)

Potential fix: 
![image](https://user-images.githubusercontent.com/10527821/115616270-af34e300-a2f8-11eb-82c9-c13bfd111fe4.png)

Potential fix is based on this line if worker.js file: 
![image](https://user-images.githubusercontent.com/10527821/115616437-e0151800-a2f8-11eb-947c-d89e72774bd5.png)


Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension): not hosted

handler.on("GetPageIndex", function wphSetupGetPageIndex({ ref }) {
      const pageRef = Ref.get(ref.num, ref.gen);
      return pdfManager.ensureCatalog("getPageIndex", [pageRef]);
    });
    handler.on("GetDestinations", function wphSetupGetDestinations(data) {
      return pdfManager.ensureCatalog("destinations");
    });
    handler.on("GetDestination", function wphSetupGetDestination(data) {
      return pdfManager.ensureCatalog("getDestination", [data.id]);
    });
    handler.on("GetPageLabels", function wphSetupGetPageLabels(data) {
      return pdfManager.ensureCatalog("pageLabels");
    });
    handler.on("GetPageLayout", function wphSetupGetPageLayout(data) {
      return pdfManager.ensureCatalog("pageLayout");
    });
    handler.on("GetPageMode", function wphSetupGetPageMode(data) {
      return pdfManager.ensureCatalog("pageMode");
    });
    handler.on("GetViewerPreferences", function (data) {
      return pdfManager.ensureCatalog("viewerPreferences");
    });
    handler.on("GetOpenAction", function (data) {
      return pdfManager.ensureCatalog("openAction");
    });
    handler.on("GetAttachments", function wphSetupGetAttachments(data) {
      return pdfManager.ensureCatalog("attachments");
    });
    handler.on("GetJavaScript", function wphSetupGetJavaScript(data) {
      return pdfManager.ensureCatalog("javaScript");