Running the external window will take down the central widget as
well, cause calling m_externalWindow->setCentralWidget(m_centralWidget)
will pass ownership to the window. Calling delete on the central
widget inside the plugin constructor will crash, so set it to NULL.

Change-Id: I88ddde68a1ca3a47182de7c19847808738431149
Reviewed-by: Karsten Heimrich <karsten.heimrich@nokia.com>