What does it mean "weakly-referenced object no longer exists"?

I am running a Python code and I get the following error message:

Exception exceptions.ReferenceError: 'weakly-referenced object no longer exists' in <bound method crawler.__del__ of <searchengine.crawler instance at 0x2b8c1f99ef80>> ignored

Does anybody know what can it means?

P.S. This is the code which produce the error:

import sqlite

class crawler:

  def __init__(self,dbname):
    tmp = sqlite.connect(dbname)
    self.con = tmp.cursor()

  def __del__(self):
    self.con.close()

crawler =  crawler('searchindex.db')