message: Fix return value of GetTriggerClass()
diff: @@ -89,7 +89,7 @@ public :
  // trigger class selection
  void SetTriggerClass(const Char_t *triggerClass) { fTriggerClass = triggerClass; }
  - const Char_t* GetTriggerClass() const { return fTriggerClass.Data(); }
  + const Char_t* GetTriggerClass() const { return fTriggerClass.IsNull()?NULL:fTriggerClass.Data(); }
  // use track vertex
  void SetUseTrackVertex(Bool_t trackVtx = kTRUE) { fUseTrackVertex = trackVtx; }
  