The error displays even when there are no spacers in the design. The object class viewer is on the left for reference, but there are no spacers in the file.
I used a spacer previously but removed it. However, I'm still getting this error, even when I have added layouts to the main window/central widget.
When I closed and re-opened QTDesigner, the file seemed to work normally again. Most likely just the IDE messing up after being open a while.