docs: def _check_validity(self):
          """Checks the validity of the state element's properties
  
          Some validity checks can only be performed by the parent. Thus, the existence of a parent and a check
          function must be ensured and this function be queried.
  
          :return: validity and messages
          :rtype: bool, str
          """
          from rafcon.core.states.state import State
  
          if not self.parent:
              return True, "no parent"
          if not isinstance(self.parent, State):
              return True, "no parental check"
          return self.parent.check_child_validity(self)