mysterious message

Anyway I handled that using try/except ....but original problem is there.

below is my code..(with solution)

error fires at highlighted log message line

class Tabs():
  def __init__(self,tab=None):
    self.tab = tab
  
  def tabExists(self):
    success = False
    while not success:
      try:
        Aliases.......MDIClient.RefreshMappingInfo()
        tabCount = Aliases...........MDIClient.ChildCount 
        if tabCount ==0 :
          return False
        for i in range (0,tabCount):
          childtab = Aliases.................MDIClient.Child(i)
          if self.tab in childtab.Text:
            Log.Message(childtab.Text+" ..exists")
            return True
        else:
          return False
        success = True   # <--this will never excute, just in case 
      except RuntimeError:
        Log.Message(RuntimeError)
        success =False

Un fortunatly can not debug  since it dosn't occure every time ....

once in 5 /6 times.