title: get_form in ModelAdmin causing TypeError
body: <p>This question refers to the issue I was having <a href="https://stackoverflow.com/questions/58897504/how-to-pass-kwargs-from-modeladmin-to-modelform">here</a> which has not been solved. I have created a new simplified version of the <code>MemoForm</code> called <code>AdminMemoForm</code> to help me isolate the issue. After creating the new version of the form I have come to find that the <code>get_form</code> method is what is causing the <code>TypeError: MemoForm object not callable.</code> Here is a snippet of the new code that is throwing the error:</p>
  
  <p><strong>Forms.py</strong>:</p>
  
  <pre><code>class AdminMemoForm(forms.ModelForm):
      """
      Memo creation form, related to:
  
      :model: 'memos.Memo',
      """
      class Meta:
          model = Memo
          fields = (
              'title',
              'content',
              'important',
              'word_file',
              'receiver',
              'read',
              'unread',
          )
  </code></pre>
  
  <p><strong>Admin.py</strong>:</p>
  
  <pre><code>class CustomMemoAdmin(admin.ModelAdmin):
      form = AdminMemoForm
  
      def get_form(self, request, obj=None, **kwargs):
          form = super().get_form(request, obj, **kwargs)
      #     if not request.user.is_superuser:
      #         self.fields = (
      #             'title',
      #             'content',
      #             'important',
      #             'receiver',
      #             'read',
      #             'unread',
      #             'word_file',
      #         )
      #     self.filter_horizontal = ('casino',)
          return form()
  </code></pre>
  
  <p>The full error:</p>
  
  <pre><code>TypeError at /admin/memos/memo/add/
  'MemoForm' object is not callable
  </code></pre>
  
  <p>Another question is why is this error showing the old form <code>MemoForm</code> and not the new <code>AdminMemoForm</code>? It is registered: <code>admin.site.register(Memo, CustomMemoAdmin)</code></p>
  
  <p>This is probably simple but I don't get why this error is arising. Any help would be appreciated.</p>