Fixed several problems with uninitialized propertyModel_ in property viz...

Fixed several problems with uninitialized propertyModel_ in property viz plugin, such as the crash in slotAllCleared.
refs #1269

git-svn-id: http://www.openflipper.org/svnrepo/OpenFlipper/branches/Free@15834 383ad7c9-94d9-4d36-a494-682f7c89f535