how to create dynamic dialogs:
QUiLoader uiLoader;
QFile file("sortdialog.ui");
QWidget *sortDialog=uiLoader.load(&file);
if(sortDialog)
{
...
}
how to access the form's child widgets:
we can use QObject::findChild<T>():
QComboBox *primaryColumnCombo=sortDialog->findChild<QComboBox*>("primaryColumnCombo");
if(primaryColumnCombo)
{
...
}

Pay attention:
If we want to use QUiLoader class,we must add this line to the application's .pro file:
CONFIG +=uitools