I've created the button cancelButton. Then I've created function-slot on_cancelButton_clicked() for it using context menu item Go to slot. But, after a while, I've deleted this button (it wasn't longer needed), and tried to delete that function, but compiler gives error undefined reference to <class_name>::on_cancelButton_clicked().
I've made cleanup of build folder (and removed it), checked for some lines in ui file that can point to this function, but i didn't found anything.
Solution is simple. I didn't remove the declaration of <class_name>::on_cancelButton_clicked() from header file. (by Ammar, posted here)