@sudarshankotian Thanks for letting me know about these reports! Indeed, there are some PHP errors generated when the CSS/JS manager is, but they are harmless notice errors and do not affect the functionality of the website.
Now, I agree they could be annoying (especially if you are a developer and you have set PHP to print even notice errors on pages, for debugging purposes) and I’ve released a fix.
You can avoid these errors if you’re using the development version that was recently updated: https://www.assetcleanup.com/docs/how-to-get-the-development-version-of-asset-cleanup/