Python django model object showing None and null in template

Question

I have this table.

class MyTable(BaseModel):
    key = m.CharField(max_length=20,null=False,unique=True)
    pre_json = m.JSONField(blank=True, null=True)
    post_json = m.JSONField(blank=True, null=True)

And I use this model through ListView/UpdateView

list view is here.

class MyTableListView(LoginRequiredMixin, ListSearchView):
    model = MyTable
    form_class = MyTableForm
    template_name = 'mytable_list.html'

class MyTableForm(forms.ModelForm):
    key = forms.CharField(required=True)
    pre_json = forms.JSONField
    post_json = forms.JSONField

    class Meta:
       model = sm.ScenarioWorker
       fields = ["key","key","pre_json","post_json"]

in list html template

{{obj.pre_json}}

in edit html template

{% render_field form.pre_json class="form-control"  %}

Upper one shows None in template

Lower one shows null in textarea as placeholder.

Why this two are shown? or can I erase these?

I want to stop this.