I am running Theia at Gitpod. They directed me here.
visx extension here:
(https://marketplace.visualstudio.com/items?itemName=tintinweb.vscode-inline-bookmarks)
Fails with this:

@rholland thank you for the issue, do you have additional information that may be useful to help identify the problem?
I installed the extension and it worked for me, it'd be good in the future to keep the issue template so we can better help:
@akosyakov I added the needs more info label since the extension worked for me, but it looks like my GitHub comment was gone.
@vince-fugnitto ah, thank you for confirming. I think Gitpod Theia needs update.
@vince-fugnitto How are you installing? I just tried again and got the same message.
Thanks.
Gitpod installs from here if you want to take a look at the setup files.
https://github.com/RivetCalc/rivetcalctemplate-001
The extension is here:
https://github.com/tintinweb/vscode-inline-bookmarks
I tried to install by drag and drop the vsix file.
Here is the console output:

I'm installing by referencing the plugin in the package.json and I'm using Theia from sources and not Gitpod. It may be a Gitpod specific issue as Anton previously mentioned.
@vince-fugnitto Thanks.
I rebuilt the project and it loads now without an error notification and shows bookmark icons in the gutter. But it does not list the bookmarks in the explorer pane. I will correspond with the author on that.