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

*
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import { Dict, Name, Ref } from "../../src/core/primitives.js";
import { incrementalUpdate, writeDict } from "../../src/core/writer.js";
import { bytesToString } from "../../src/shared/util.js";
import { StringStream } from "../../src/core/stream.js";
describe("Writer", function () {
  describe("Incremental update", function () {
    it("should update a file with new objects", function () {
      const originalData = new Uint8Array();
      const newRefs = [
        { ref: Ref.get(123, 0x2d), data: "abc\n" },
        { ref: Ref.get(456, 0x4e), data: "defg\n" },
      ];
      const xrefInfo = {
        newRef: Ref.get(789, 0),
        startXRef: 314,
        fileIds: ["id", ""],
        rootRef: null,
        infoRef: null,
        encrypt: null,
        filename: "foo.pdf",
        info: {},
      };
      let data = incrementalUpdate({ originalData, xrefInfo, newRefs });