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 (enumerableProperties.length) {
        throw new Error(
          "The `Array.prototype` contains unexpected enumerable properties: " +
            enumerableProperties.join(", ") +
            "; thus breaking e.g. `for...in` iteration of `Array`s."
        );
      }
      
      
      
      if (
        (typeof PDFJSDev === "undefined" || PDFJSDev.test("SKIP_BABEL")) &&
        typeof ReadableStream === "undefined"
      ) {
        throw new Error(
          "The browser/environment lacks native support for critical " +
            "functionality used by the PDF.js library (e.g. `ReadableStream`); " +
            "please use a `legacy`-build instead."
        );
      }
    }
    var docId = docParams.docId;
    var docBaseUrl = docParams.docBaseUrl;
    var workerHandlerName = docParams.docId + "_worker";
    var handler = new MessageHandler(workerHandlerName, docId, port);
    
    
    handler.postMessageTransfers = docParams.postMessageTransfers;
    function ensureNotTerminated() {