Not sure what happened but I guess I broke the category stuffs inside Category tab. Also, this dialog window is somewhat confusing.
Also, for goodness sake, please fix the “Clear” hover state… it’s annoying! 
Hi Aaron,
This message appears when you try to delete all the nodes from the category tree. As per design for the display purpose, there is always a node required in the category tree. This node could be an empty node.
Thank you for providing the feedback on the “Clear” hover …
Regards,
Shweta Patel.
That’s no problem. Users, like me, doesn’t understand what “node” means, so perhaps explain it like in 2nd grade level term? That will be super easy for all of us 
You’re welcome about the feedback 
Sorry!
So the categories are displayed on the screen using the pattern of a tree. Where there is a root and branches(also called nodes). Each node indicates the category that you can see( Which can further have sub-branches / children / child nodes ). It displays a “+” on the left if it has further children/child nodes.
The object which is used in the UI to display the categories does not allow zero nodes and so there should be at least one which could be empty.
Hope I was better this time :).
Regards,
Shweta Patel.