Django admin TabularInline with custom form

I want to use TabularInline for my model-admin with a custom form and extra=3. But the problem is that when I hit save in my admin page, the form gives validation error on empty extras that I didn't use and can't save the models I wanted to create. I read the docs in django and can't quite understand the solution to my problem.

The reason I want to use a custom form is that I'm using custom widgets for some of the fields. So there may be a better solution for my needs than using a custom form which I'm not aware of. I'm using Django v2.0 and here's the code for my admin:

class BingilForm(forms.ModelForm):
    class Meta:
        model = Bingil
        fields = '__all__'

        widgets = {
            'position_top': RangeInput(),
            'position_left': RangeInput(),
            'background_color': ColorInput(),
            'foreground_color': ColorInput(),
        }

class InlineVideoBingil(admin.TabularInline):
    model = Bingil
    fk_name = 'video'
    form = BingilForm
    extra = 3
    show_change_link = True