Hello,
I'm having a issue with Lots/Serial Numbers in odoo 11 , i have third part module & was wroking fine 

but now when i wanna create new  Lots/Serial Numberi get this err :

Traceback (most recent call last):
File "/opt/odoo/odoo/http.py", line 653, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "/opt/odoo/odoo/http.py", line 312, in _handle_exception
raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
File "/opt/odoo/odoo/tools/pycompat.py", line 87, in reraise
raise value
File "/opt/odoo/odoo/http.py", line 695, in dispatch
result = self._call_function(**self.params)
File "/opt/odoo/odoo/http.py", line 344, in _call_function
return checked_call(self.db, *args, **kwargs)
File "/opt/odoo/odoo/service/model.py", line 97, in wrapper
return f(dbname, *args, **kwargs)
File "/opt/odoo/odoo/http.py", line 337, in checked_call
result = self.endpoint(*a, **kw)
File "/opt/odoo/odoo/http.py", line 939, in __call__
return self.method(*args, **kw)
File "/opt/odoo/odoo/http.py", line 517, in response_wrap
response = f(*args, **kw)
File "/opt/odoo/addons/web/controllers/main.py", line 935, in call_kw
return self._call_kw(model, method, args, kwargs)
File "/opt/odoo/addons/web/controllers/main.py", line 927, in _call_kw
return call_kw(request.env[model], method, args, kwargs)
File "/opt/odoo/odoo/api.py", line 697, in call_kw
return call_kw_model(method, model, args, kwargs)
File "/opt/odoo/odoo/api.py", line 682, in call_kw_model
result = method(recs, *args, **kwargs)
File "/opt/berger_addons/serial_unique_warning/models/stock_production_lot.py", line 75, in create
partner.write({'category_id': [( 4,tag_id.id)]})
File "/opt/odoo/odoo/fields.py", line 2590, in __get__
raise ValueError("Expected singleton: %s" % record)
ValueError: Expected singleton: res.partner.category(36, 59)

I have no idea how to solve this. Can anyone help ? Thanks in advance