b1802a164b added handling for a parent to
become focus proxy of a child. The respective 'else if' branch didn't
check whether setFocusProxy() was called with a nullptr argument.
This patch adds the missing nullptr check.
It also adds functionality to tst_QWidget::tabOrderComboBox() to test
the removal of a focus proxy, as well as the complete removal of an
element from the focus chain.
Change-Id: I4cb865b9ac4497fc5e2595910738fb77694f5837
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
72d6768ec1