Promotion in QtCreator is simply a way to tell QtCreator you are using a subclass of a Qt witdget, so that it knows to include the correct header for your widget, that is all.
Where is the implementation of the header you posted?
The errors are linking error, which means probably the lib or object files of your custom widgets is not found (have a look a previous errors).
Make sure the link paths are correctly set to include the lib of your custom widgets (if you are indeed use a lib).