Since chrome blocks the redirection to URLs that are not properly formatted (AFAIK they simply need to start with http:// or https://), there should be some kind of validation in the popup telling the user that the URL is not valid. Depending on the implementation details it might make sense to redirect the user to an error page if they try to access an incorrect URL instead of opening a blank page with the address of the extension id.
Since chrome blocks the redirection to URLs that are not properly formatted (AFAIK they simply need to start with http:// or https://), there should be some kind of validation in the popup telling the user that the URL is not valid. Depending on the implementation details it might make sense to redirect the user to an error page if they try to access an incorrect URL instead of opening a blank page with the address of the extension id.