Adding an empty category returns a "This document already exists" message

When adding a new blogpost, in the category add form at the bottom, there are 2 issues:

• the "Add" button can be easily mistaken by the button to add the whole post, if the page is not completely scrolled and the post save buttons are not visible. A better label could be "Add Category", to suggest that it's for the category form only
• if the "Add" button is hit and no category is entered, the alert says: "This category already exists", when in fact no category was attempted to be added. Maybe a warning message that the form should be filed first