The issue with hidden field

Problem/Motivation

I tried to have a view parent which only brings in content from other two views. When adding the combine field into the view it's being added as hidden by default making it impossible to only have content from other views without the local view outputting something.

Steps to reproduce

Create two views to be used as sources for the main view to combine.
Add a parent view with these two views as the source in two combine fields.
Can't use the parent view because these fields are hidden therefore it complains that it must have at least one visible field.

Proposed resolution

Do not hide these fields by default so we can have a holding view of multiple source views.