You open up the Chrome extension on a blank Google page, but it automatically redirects you to https://www.getcloudapp.com/chrome-welcome.
Unfortunately, this is a limit of the Chrome extension. When using Google pages (Google.com, a Google home page, etc), you are unable to open Chrome extensions.
This should not occur on non-Google pages. When it's not a real webpage (like the new tab dialog, or settings, or etc.) we cannot screenshot the open page containing an empty URL to include the CloudApp extension.