access on main window from anothe window (e.g Dialog)

[Malek]

Good evening everybody...

My Question as simple as in the title !!
I try to write in line edit witch is in the main window,while my button exist on another window (dialog window).

When I try to make that, I put in the event handler the dialog button:

MainWindow::ui->lineEdit_1->text() = :ui->lineEdit_2->text() ;

MainWindow::ui->lineEdit_1->text() = :ui->lineEdit_2->text() ;

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while :lineEdit_1 is in the main window and :lineEdit_2 is in the dialoge.

but the honorable compiler tell me that :
1) Object missing in reference to 'MainWindow::ui' .............in mainwindow.h
2)from this location .............in dialog.cpp

I have search about this, but I didn't understand the meant.Let alone most of what I found was about multi threading not about forms.

Thank you.