Fix for WebviewPanelSerializer not working in Theia

[vatsal-uppal-1997]

What it does

Fixes #12834

For a detailed root cause analysis, refer to this comment

How to test

1. Clone the test repository from issue WebviewPanelSerializer not working in Theia #12834
2. cd into the "myext" folder and create a packaged vscode plugin by running the "vsce package" command
3. Copy the packaged vsix file into this issue's git repository and paste it into the "plugins" folder
4. Run the application and see if you're able to reproduce issue WebviewPanelSerializer not working in Theia #12834

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[jfaltermeier]

Thanks you, lgtm. I've tested with the reproducer on the ticket and it is working for me.