Prevent User from proceeding if they do not select a template


I use the New Blog Templates plugin for my Multisite website. When the users do not select a template, e.g when they hit a ‘return button’ after they enter their site title and do not see the template selection, is there a way we can add some codes to prompt the user that he cannot proceed unless he select a template? If it is possible, can you advise the codes to add and how.

Thank you