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 (terminated) {
        throw new Error("Worker was terminated");
      }
    }
    function startWorkerTask(task) {
      WorkerTasks.push(task);
    }
    function finishWorkerTask(task) {
      task.finish();
      var i = WorkerTasks.indexOf(task);
      WorkerTasks.splice(i, 1);
    }
    async function loadDocument(recoveryMode) {