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

}
          if (newRefs.length === 0) {
            
            return stream.bytes;
          }
          const xfa = (acroForm instanceof Dict && acroForm.get("XFA")) || [];
          let xfaDatasets = null;
          if (Array.isArray(xfa)) {
            for (let i = 0, ii = xfa.length; i < ii; i += 2) {
              if (xfa[i] === "datasets") {
                xfaDatasets = xfa[i + 1];
              }
            }
          } else {
            
            warn("Unsupported XFA type.");
          }
          let newXrefInfo = Object.create(null);
          if (xref.trailer) {
            
            const infoObj = Object.create(null);
            const xrefInfo = xref.trailer.get("Info") || null;
            if (xrefInfo instanceof Dict) {
              xrefInfo.forEach((key, value) => {
                if (isString(key) && isString(value)) {
                  infoObj[key] = stringToPDFString(value);
                }
              });
            }
            newXrefInfo = {