SOLUTION 1: IN CODE
I discovered that I can make it 'work' if I change (in Ui_Dialog):
self.scrollArea.setWidget(self.scrollAreaWidgetContents)
to
self.scrollArea.setWidget(self.label)
I have no idea why QtDesigner would have created code that doesn't work, and I don't fully understand WHY that code didn't work, when this seems to. self.label had self.scrollAreaWidgetContents as its parent, so the connections seem to be there.
Accordingly, I'm guessing that I'm doing something wrong in QtDesigner that's preventing this from exporting code that works.
SOLUTION 2: IN QT DESIGNER
Okay, so I discovered that I can make it work through QtDesigner if I do the following:
(1) Create a ScrollArea [cannot set the layout of the ScrollArea at this point]
(2) Place a Label inside the ScrollArea
(3) Right click on the ScrollArea, set its layout (e.g., Horizontal Layout)
(4) Export
This seems to work just fine. HOWEVER, it does NOT work if I do the following:
(1) Create a ScrollArea
(2) Place a Layout in the ScrollArea (e.g., Horizontal Layout)
(3) Place a Label inside the Layout
(4) Export
I'm not exactly sure what's going on here, but, nevertheless, these are two solutions to the problem.