old layout item remain on widget

[lllturtle]

hello everyone ,
i've got some problem about change the layout item's.
i'm using : qt 4.7 in winidows xp sp3

i made a simple program , with 2 button's change a mainLayout item
the item compositing by several label , and i put some word in corner ,
after change layout (from 4 label to 2 label) some word still exist on window
and part of button covered by the old label .

i put whole project and picture in attachments , any suggestion would be appreciate

thanks
and sorry for my poor english
---------
another problem is after i delete the layout item
i always got the following msg , but everything keep going

QCoreApplication:: postEvent: Unexpected null receiver
QLayout::addChildLayout: layout "" already has a parent

code about delete layout item

QLayoutItem *child;
    while ((child = mainLayout->takeAt(0)) != 0) {
        child->widget()->deleteLater();
    }
    delete child;

QLayoutItem *child;
    while ((child = mainLayout->takeAt(0)) != 0) {
        child->widget()->deleteLater();
    }
    delete child;

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