Hi @anon72326184,
UI kit does not support a cleanup function. Note that while UI kit may look similar to react, it is not the same as react.
https://developer.atlassian.com/platform/forge/ui-kit-hooks-reference/#useeffect
Also, from your code snippet, you need to be careful with setTimeout usage as there are some caveats: SetInterval and SetTimeout usage with Forge UI - #2 by SamSmyth