If I start with an empty container there is no "plus" button to add items. The button will only appear if I add an item by loading a preset. See attached image.
When you add the widget in the screen and you open the Kwgt editor, if you want to add an empty container you need to add one touching in the sheet icon in the top right of the editor.
I recommend to you to start from zero with a new widget in your screen and when first appear the editor you have the option to add an empty container, like I said.👍