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

pdfManager.onLoadedStream(),
          pdfManager.ensureCatalog("acroForm"),
          pdfManager.ensureDoc("xref"),
          pdfManager.ensureDoc("startXRef"),
        ];
        for (let pageIndex = 0; pageIndex < numPages; pageIndex++) {
          promises.push(
            pdfManager.getPage(pageIndex).then(function (page) {
              const task = new WorkerTask(`Save: page ${pageIndex}`);
              startWorkerTask(task);
              return page
                .save(handler, task, annotationStorage)
                .finally(function () {
                  finishWorkerTask(task);
                });
            })
          );
        }
        return Promise.all(promises).then(function ([
          stream,
          acroForm,
          xref,
          startXRef,
          ...refs
        ]) {
          let newRefs = [];
          for (const ref of refs) {
            newRefs = ref
              .filter(x => x !== null)
              .reduce((a, b) => a.concat(b), newRefs);