better handling for the warning for the "refresh" layers widget?

[gioman]

Create a project with no OGC layers.

Add the "refresh layers" widget.

On preview you'll get

"Refresh layers component added, but it has no layers configured to be refreshed".

So you go in the widget options but there are no layers, and (of course) no way to add them.

If OGC layers are missing and the widget added maybe is better to silently disable it? just an idea.