python - AttributeError at ....... 'NoneType' object has no attribute 'is_valid' while developing django model forms -

i error while saving modelform database.

"attributeerror @ /giris/markayenikaydet/, 'nonetype' object has no attribute is_valid "

what may cause ? stuck. related modelform generation or related not including ant function.

""""

model...

from __future__ import unicode_literals django.db import models datetime import datetime django.urls import reverse django.utils.translation import gettext _   class marka(models.model):     marka_adi = models.charfield(max_length=200)     def __str__(self):         return(self.marka_adi) 

url...

from django.views.generic import redirectview django.conf.urls import include, url . import views django.utils.translation import gettext _   urlpatterns = [     url(r'^$', views.index, name='index'),     url(r'^demirbas/$', views.demirbaslistview.as_view(), name='demirbas'),     url(r'^demirbas/(?p<pk>\d+)$', views.demirbasdetailview.as_view(), name='demirbas-detail'),     url(r'^proje/$', views.projelistview.as_view(), name='proje'),     url(r'^proje/(?p<pk>\d+)$', views.projedetailview.as_view(), name='proje-detail'),     url(r'^marka/$', views.markalistview.as_view(), name='marka'),     url(r'^marka/(?p<pk>\d+)$', views.markadetailview.as_view(), name='marka-detail'),     url(r'^marka/duzeltsil/$', views.markaduzeltsil, name='markaduzeltsil'),     url(r'^marka/ekle/$', views.markaekle, name='markaekle'),     url(r'^kategori/$', views.kategorilistview.as_view(), name='kategori'),     url(r'^kategori/(?p<pk>\d+)$', views.kategoridetailview.as_view(), name='kategori-detail'),     url(r'^musteri/$', views.musterilistview.as_view(), name='musteri'),     url(r'^musteri/(?p<pk>\d+)$', views.musteridetailview.as_view(), name='musteri-detail'),     url(r'^marka/create/$', views.markacreate.as_view(), name='marka_create'),     url(r'^marka/(?p<pk>\d+)/update/$', views.markaupdate.as_view(), name='marka_update'),     url(r'^marka/(?p<pk>\d+)/delete/$', views.markadelete.as_view(), name='marka_delete'),     url(r'^markayenikaydet/$', views.markayenikaydet, name='markayenikaydet'),     ] 

form....

from django import forms django.forms import modelform giris.models import marka django.utils.translation import gettext _    """from crispy_forms.helper import formhelper crispy_forms.layout import submit, layout, field crispy_forms.bootstrap import (     prependedtext, prependedappendedtext, formactions)"""  def markagirisform(modelform):     class meta:             model = marka             fields = '__all__'             labels = { 'marka_adi': _('marka adı')}             help_texts = { 'marka_adi': _('lüften marka adı giriniz..')} 

view..

class markacreate(createview):     model = marka     fields = '__all__'  class markaupdate(updateview):     model = marka     fields = '__all__'  class markadelete(deleteview):     model = marka     success_url = reverse_lazy('marka_del')   django.http import httpresponseredirect django.shortcuts import render .forms import markagirisform django.utils.translation import gettext _    def markayenikaydet(request):     if request.method == 'post':         form = markagirisform(request.post)         if  form.is_valid():             marka_ad_gecici = request.post.get('marka_adi')             marka_obj = marka(marka_adi = marka_ad_gecici)             marka.save()     else:         form = markagirisform()     return render(request, '/giris/addbook.html', {'form': form}) 

template...

{% extends "base_generic.html" %}  {% block content %}  <form action="{% url 'markayenikaydet' %}" method="post">     {% csrf_token %}     <table>     {{ form.as_table }}     </table>     <input type="submit" value="kaydet" />  </form> {% endblock %}