Actually I have two choices but the upper one (haha.h) is does not exist, and I can't find the way to delete it. I am beginner in Qt, so someone can help me?
You can remove it manually, just open the MainWindow.ui with an editor and remove the xml entry of your promoted class. You will find something like this:
<customwidget>
<class>haha</class>
<extends>QLabel</extends>
<header>haha.h</header>
<container>0</container>
</customwidget>
Check the counter, if it higher than 0, then you have to demote all the promoted widgets. Another way its to try to promote a simple QWidget an then remove it from the Qt Creator menu,