Cannot set layout for a widget form

[GunkutA]

Hello I cannot set a layout to the widget form I created. That is the related screenshot:

How can I set a layout to it?

[Bonnie]

You must add a child widget to it first, then you can set the layout.
You can delete the added widget after that.

[JonB]

@GunkutA
Just so you know: what @Bonnie has said is correct, and it's about one of the least obvious features imaginable in Qt Creator! You are not alone, it took me ages to realize this is how it works!