"An extra protection against an erroneous "Gui.Prompt" value"

 the use case was reported via
 http://root.cern.ch/phpBB2/viewtopic.php?p=17124#17124 )


git-svn-id: http://root.cern.ch/svn/root/trunk@17206 27541ba8-7e3a-0410-8455-c3a389f83636