Perhaps you compiled "xyz" in debug mode and application in release mode?
Man, this stupid thing just nailed me for two days. When I set up my release mode build in Visual Studio, I copied the list of libraries from the debug settings, then carefully edited all of the "d" suffixes off. On all but two libraries...

For the first day, I couldn't figure out what was happening - the app wouldn't even get into main(). You could see it start in Task Manager, then it immediately exited. No error message, no crash, nothing. Finally thought to link it with debug turned on, and saw the "QPixmap: Must construct a QGuiApplication first" message. Spent a wasted hour looking for QPixmap, and of course there aren't any defined as static in my code. Finally came across this thread and thought, "I wonder if I've been stupid..."

So, for anyone else using Visual Studio or the MSVC compilers to build your Qt projects, you can't mix debug and release libraries or debug libraries and a release executable. If you get the incomprehensible "QPixmap: Must construct..." error, it means that's what you've done.