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

const waitOn = [];
      if (pdfManager) {
        pdfManager.terminate(new AbortException("Worker was terminated."));
        const cleanupPromise = pdfManager.cleanup();
        waitOn.push(cleanupPromise);
        pdfManager = null;
      } else {
        clearPrimitiveCaches();
      }
      if (cancelXHRs) {
        cancelXHRs(new AbortException("Worker was terminated."));
      }
      WorkerTasks.forEach(function (task) {
        waitOn.push(task.finished);
        task.terminate();
      });
      return Promise.all(waitOn).then(function () {
        
        
        handler.destroy();
        handler = null;
      });
    });
    handler.on("Ready", function wphReady(data) {
      setupDoc(docParams);
      docParams = null; 
    });
    return workerHandlerName;
  }
  static initializeFromPort(port) {