don't use delete but instead use deleteLater()
and you'll need to remove the old widget first:
void PlainView::addComponent(QWidget *widget)
{
qDebug() << _splitOne->widget(1);
QWidget *old = _splitOne->widget(1);
// deparenting removes the widget from the gui
old->setParent(0);
//delete current widget on index 1
old->deleteLater()
//add new widget on index 1
_splitOne->insertWidget(1,widget);
widget->show();
qDebug() << _splitOne->widget(1);
}