Exception on configuration serialization

[mohierf]

Got this error:

[2019-06-06 08:39:10] CRITICAL: [arbiter-master.alignak.daemon] -----
Exception: 'str' object has no attribute 'serialize'
Back trace of the error:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/alignak/daemons/arbiterdaemon.py", line 2149, in main
    self.do_main_loop()
  File "/usr/local/lib/python3.6/site-packages/alignak/daemon.py", line 1025, in do_main_loop
    self.do_before_loop()
  File "/usr/local/lib/python3.6/site-packages/alignak/daemons/arbiterdaemon.py", line 1560, in do_before_loop
    self.configuration_dispatch()
  File "/usr/local/lib/python3.6/site-packages/alignak/daemons/arbiterdaemon.py", line 1492, in configuration_dispatch
    self.dispatcher.prepare_dispatch()
  File "/usr/local/lib/python3.6/site-packages/alignak/dispatcher.py", line 659, in prepare_dispatch
    s_conf_part = serialize(realm.parts[cfg_part.instance_id])
  File "/usr/local/lib/python3.6/site-packages/alignak/misc/serialization.py", line 53, in serialize
    'content': obj.serialize()
  File "/usr/local/lib/python3.6/site-packages/alignak/objects/config.py", line 999, in serialize
    res[strclss] = serialize(items)
  File "/usr/local/lib/python3.6/site-packages/alignak/misc/serialization.py", line 53, in serialize
    'content': obj.serialize()
  File "/usr/local/lib/python3.6/site-packages/alignak/objects/item.py", line 1287, in serialize
    res['templates'][key] = item.serialize()
  File "/usr/local/lib/python3.6/site-packages/alignak/objects/contact.py", line 223, in serialize
    res[prop] = [c.serialize() for c in getattr(self, prop)]
  File "/usr/local/lib/python3.6/site-packages/alignak/objects/contact.py", line 223, in <listcomp>
    res[prop] = [c.serialize() for c in getattr(self, prop)]
AttributeError: 'str' object has no attribute 'serialize'

This with a contact configuration with several notification commands