i have this problem , i don't know what is it 

Erreur:
Odoo Server Error

Traceback (most recent call last):
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\api.py", line 1039, in get
value = self._data[key][field][record._ids[0]]
KeyError: <odoo.models.NewId object at 0x06EE6090>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\fields.py", line 981, in __get__
value = record.env.cache.get(record, self)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\api.py", line 1041, in get
raise CacheMiss(record, field)
odoo.exceptions.CacheMiss: ('fleet.vehicle(<odoo.models.NewId object at 0x06EE6090>,).v_neuf', None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 656, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 314, in _handle_exception
raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\tools\pycompat.py", line 87, in reraise
raise value
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 698, in dispatch
result = self._call_function(**self.params)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 346, in _call_function
return checked_call(self.db, *args, **kwargs)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\service\model.py", line 97, in wrapper
return f(dbname, *args, **kwargs)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 339, in checked_call
result = self.endpoint(*a, **kw)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 941, in __call__
return self.method(*args, **kw)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\http.py", line 519, in response_wrap
response = f(*args, **kw)
File "c:\program files (x86)\odoo 12.0\server\odoo\addons\web\controllers\main.py", line 962, in call_kw
return self._call_kw(model, method, args, kwargs)
File "c:\program files (x86)\odoo 12.0\server\odoo\addons\web\controllers\main.py", line 954, in _call_kw
return call_kw(request.env[model], method, args, kwargs)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\api.py", line 749, in call_kw
return _call_kw_multi(method, model, args, kwargs)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\api.py", line 736, in _call_kw_multi
result = method(recs, *args, **kwargs)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\models.py", line 5513, in onchange
for name in nametree
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\models.py", line 5514, in <listcomp>
if name not in done and snapshot0.has_changed(name)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\models.py", line 5416, in has_changed
return self[name] != record[name]
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\models.py", line 5117, in __getitem__
return self._fields[key].__get__(self, type(self))
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\fields.py", line 987, in __get__
self.determine_draft_value(record)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\fields.py", line 1109, in determine_draft_value
self._compute_value(record)
File "C:\Program Files (x86)\Odoo 12.0\server\odoo\fields.py", line 1043, in _compute_value
getattr(records, self.compute)()
File "c:\program files (x86)\odoo 12.0\server\odoo\addons\fleet\models\fleet_vehicle.py", line 105, in _compute_valeur_Vinale
val_diff = (car_value * taux_remise )
TypeError: unsupported operand type(s) for *: 'Float' and 'Float'