Error LNK2019. How to eliminate?

Question

Qt

Good time of the day!
For the second day I can not cope with the error:

main.obj:-1: error: LNK2019: unresolved external symbol reference "public: __cdecl Backend::Backend(class QQuickItem *)" ([email protected]@[email protected]@@@Z) in main function

Good people, tell me what kind of infection this is and how to deal with it?
Files:
pastebin.com/KD1riNa5 - main.cpp
pastebin.com/jiF48PvX - backend.h
pastebin.com/TbRZDXRR - backend.cpp
pastebin.com/DSfvKx8i - main.qml
Whole project:
https://drive.google.com /file/d/0B5AyF2K9QH-FMmlVN...
Did according to this article.
Moreover, if you download the archive with its finished project, then it starts normally.
Thanks in advance!

Answer 1

restart qmake