Diagonal stripes on modal window background

I have a modal dialog window in my game that opens as a new window. It works fine, except that I am unable to set the background color. The background is dark grey diagonal stripes. I’m guessing the diagonal stripes are there due to some error condition it’s trying to tell me about.

This is the code snippet:

						SAssignNew(MainDisplayVBox, SVerticalBox)

Any ideas?