Usability: <none> category must be selected manually when creating containers

Given the case I want to start out by creating a couple of (still empty) containers, from the second container on I get an error "<Name-of-first-Container> field is required".

To create the new container, the user needs to find and open the "Categories" fold, and click the <none> entry for all already existing containers.

This could/should be pre-set to save the mouse shuffling.