This is an integration issue.
Open the attached project in Qt Creator and try to compile it - you'll get this message (see screenshot1):
> 'ui_mainwindow.h' file not found
Now, if you go ahead and add "ui_mainwindow.h" to Product's files in widgetsapp.qbs (this is what a new Qt user would likely do) and try to compile the project you'll get (see screenshot2)
> No executable specified.
Notice that there's no any additional information about the actual reason why the project cannot be compiled and the source files tree is not grayed out how it usually is if any files are missing.