Address http://bugs.kde.org/show_bug.cgi?id=338797#c7,

static_cast<UMLClassifier*>(m_umlObject) in ClassifierWidget::classifier()
may be invalid:

umbrello/widgets/classifierwidget.cpp
- In function classifier(), replace static_cast by dynamic_cast.
- In the entire class implementation, ensure return value from classifier()
  is non NULL before dereferencing.

Since I could not reproduce the crash, I leave this bug REOPENED until the
fix is verified.

CCBUG: 338797