Odoo : Error on the second creation of project deadline - python-3.x

Question

In Odoo framework, i've all projects done and created already, now im stack on projects deadline, when i want to create a deadline of projet, the first commit and operation passes successfully, but the second gives me this error below :
Traceback (most recent call last):
File "D:\Odoo 11.0\server\odoo\fields.py", line 936, in __get__
value = record.env.cache.get(record, self)
File "D:\Odoo 11.0\server\odoo\api.py", line 960, in get
value = self._data[field][record.id][key]
KeyError: <odoo.api.Environment object at 0x000000000B34F550>
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "D:\Odoo 11.0\server\odoo\http.py", line 647, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "D:\Odoo 11.0\server\odoo\http.py", line 307, in _handle_exception
raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
File "D:\Odoo 11.0\server\odoo\tools\pycompat.py", line 87, in reraise
raise value
File "D:\Odoo 11.0\server\odoo\http.py", line 689, in dispatch
result = self._call_function(**self.params)
File "D:\Odoo 11.0\server\odoo\http.py", line 339, in _call_function
return checked_call(self.db, *args, **kwargs)
File "D:\Odoo 11.0\server\odoo\service\model.py", line 97, in wrapper
return f(dbname, *args, **kwargs)
File "D:\Odoo 11.0\server\odoo\http.py", line 332, in checked_call
result = self.endpoint(*a, **kw)
File "D:\Odoo 11.0\server\odoo\http.py", line 933, in __call__
return self.method(*args, **kw)
File "D:\Odoo 11.0\server\odoo\http.py", line 512, in response_wrap
response = f(*args, **kw)
File "D:\Odoo 11.0\server\odoo\addons\web\controllers\main.py", line 872, in search_read
return self.do_search_read(model, fields, offset, limit, domain, sort)
File "D:\Odoo 11.0\server\odoo\addons\web\controllers\main.py", line 894, in do_search_read
offset=offset or 0, limit=limit or False, order=sort or False)
File "D:\Odoo 11.0\server\odoo\models.py", line 4169, in search_read
result = records.read(fields)
File "D:\Odoo 11.0\server\odoo\models.py", line 2535, in read
values[name] = field.convert_to_read(record[name], record, use_name_get)
File "D:\Odoo 11.0\server\odoo\models.py", line 4688, in __getitem__
return self._fields[key].__get__(self, type(self))
File "D:\Odoo 11.0\server\odoo\fields.py", line 940, in __get__
self.determine_value(record)
File "D:\Odoo 11.0\server\odoo\fields.py", line 1051, in determine_value
self.compute_value(recs)
File "D:\Odoo 11.0\server\odoo\fields.py", line 1007, in compute_value
self._compute_value(records)
File "D:\Odoo 11.0\server\odoo\fields.py", line 998, in _compute_value
getattr(records, self.compute)()
File "D:\Odoo 11.0\server\odoo\addons\dev\models\delais.py", line 24, in _delai_arret
domaine = ('project_ids', '=', self.project_ids).id
File "D:\Odoo 11.0\server\odoo\fields.py", line 934, in __get__
record.ensure_one()
File "D:\Odoo 11.0\server\odoo\models.py", line 4296, in ensure_one
raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: delais.delais(2, 18)
And this is the function which calculate the sum of missions number of each project, in this function i got the error. Please i didn't figure out where is the problem exactly, help me to get rid of this please, thank's for your support.
def _delai_arret(self):
domaine = ('project_ids', '=', self.project_ids).id
dict = self.env['mission.mission'].search_read([domaine], ['nbr_mission'])
# print(dict)
if not dict:
pass
else:
somme = 0
for key in dict:
# print(key['nbr_mission'])
somme = somme + key['nbr_mission'] / 30
# print('la somme est : {somme}')
self.delai_arr_mois = somme
# print(somme)