'datetime.datetime' object has no attribute 'strip' - django

Question

I created an update form, the form has 2 time field and 2 date time field, once click on submit even nothing change, I got an error "'datetime.datetime' object has no attribute 'strip'".
but there is no error when creating a new object for the same model .
modles:
time_start=models.DateTimeField(blank=True,null=True)
htime_start=models.TimeField(blank=True,null=True)
time_end = models.DateTimeField(blank=True, null=True)
htime_end=models.TimeField(blank=True,null=True)
time_require=models.DateTimeField(blank=True,null=True)
machine=models.ForeignKey(Machine,null=True, on_delete=models.SET_NULL, blank=True)
forms:
class AssignTimeForProjects(forms.ModelForm):
title = forms.CharField(disabled=True)
owner=forms.CharField(disabled=True)
time_require=forms.TimeField(disabled=True)
class Meta:
model = tb_order
# fields='__all__'
fields = ['title', 'owner', 'grade', 'quantity',
'time_require','htime_start','color',
'mould','time_start','time_end', 'htime_end' ,'machine']
def __init__(self, *args, **kwargs):
super(AssignTimeForProjects, self).__init__(*args, **kwargs)
self.fields["time_end"] = JalaliDateField(label=('تاریخ اتمام'), widget=(AdminJalaliDateWidget))
self.fields["time_start"] = JalaliDateField(label=('تاریخ شروع'), widget=(AdminJalaliDateWidget))
Html:
<label>{{ form.htime_start.label}}</label>
{% render_field form.htime_start type="time" class+="form-control timepicker" autocomplete="off" %}
<br>
<label>{{ form.time_end.label}}</label>
{% render_field form.time_end class+="form-control" autocomplete="off" %}
<br>
<label>{{ form.htime_end.label}}</label>
{% render_field form.htime_end type="time" class+="form-control" autocomplete="off" %}