Invocation QToolBox::removeItem() before MainWindow->show()

I've QToolBox with 3 items (created from QtDesigner).

If to try to remove a first item of QToolBox before MainWindow::show(), the widgets from this item still present on MainWindow (but without layout and item header). But if invoke show() before removeItem(0) - all will ok, no widgets from this items present on the screen.

Example added to comments.