Found my issue. An update to Citrix Receiver, which I use for work, isn't playing well with the Steam version of Catan Universe. I've found others reporting similar issues with Citrix and some other Steam-based games. Uninstalled Citrix and no more crashes. Unfortunately I need Citrix, so this isn't a long-term solution.
As nasenstein already mentioned, please contact our support team with further details on your system. As we can see from the screenshot, a shader error points to a graphics card or driver issue. Are you using a card according to the minimum system requirements with up-to-date drivers?
@jwm22 The developers were able to identify the problem; a hotfix is available for download. The hotfix will resolve issues with the pop-ups which caused the game to stop either when developing a city or exiting a finished game (= your case). Please reload the WebGL version to make sure that you play the most recent version of Catan Universe. You can check whether you use the correct version by verifying the version code displayed in the main menu: version 1.3.0-868e3b31e is the most recent version.