I apologize you're seeing this change in the footer of your sign up form! We have seen cases where inline and pop up forms shows "null" instead of "email messages" text in their footers. This is an experience we are continuing to troubleshoot and have tracked in your account. As a workaround we suggest changing "marketing emails" to just "emails" in your settings. Once you have made this update you will want to then deactivate and reactivate the your form.
Did this workaround work?