How to create a custom widget for a field?

Question

Django

Hello! Please help me figure it out.
I'm trying to create a custom widget for a field like in the picture below. There is a form with a function field (ModelMultipleChoiceField). I form the queryset for the field in the view and pass it through the constructor to the form. All Groups are passed to the queryset. Wrote a template for the widget. I want to give the user the opportunity to select several functions through checkboxes. I used the following code, but the form is empty, why is the render method in the custom widget not working? I think you need to pass groups to the tag that is in the widget, but how?

forms.py:

class RequirementForm(forms.ModelForm):
    function = forms.ModelMultipleChoiceField(required=True, widget=CustomTreeWidget, queryset=Group.objects.none())

    class Meta:
        model = Requirement
        fields = ('function',)

    def __init__(self, all_groups, all_user_characteristics, *args, **kwargs):
        super(RequirementForm, self).__init__(*args, **kwargs)
        self.fields['function'].queryset = all_groups  # Cписок всех Групп берутся из view

tree_widget.html:

{% for group in groups %}
    <p>{{ group }}</p>
    {% for task in group.task_set.all %}
        <p>{{ task }}</p>
        {% for function in task.function_set.all %}
            <input type="checkbox" name="option" value="{{ function }}">{{ function }}<br>
        {% endfor %}
    {% endfor %}
{% endfor %}

widgets.py:

class CustomTreeWidget(Widget):
    template_name = 'tree_widget.html'

    def render(self, name, value, attrs=None):
        template = loader.get_template(self.template_name)
        return mark_safe(template)