This is access to GUI element, as you said. My code (subclassing) looks almost the same, and it doesn't work, because I have wrong GUI element access.
p.s.
I changed contents of my main.cpp file (it was wrong code in it) and it works.
edit. I mean that it doesn't crash, window is displayed but with no functionality. If I added ui. prefix in refering to GUI element would it be ok?