Bug Description

After upgrading to django-select2 v8.4.1, ModelSelect2MultipleWidget and other HeavySelect2Mixin subclasses throw AttributeError: 'ModelSelect2MultipleWidget' object has no attribute 'uuid' when rendering. This appears to be a regression introduced by the security fix in v8.4.1

The issue occurs because uuid generation was moved from __init__ to build_attrs in v8.4.1, but _get_cache_key is called earlier in the render flow (render() → set_to_cache() → _get_cache_key())

Steps to Reproduce

1- Install django-select2 v8.4.1
2- Create any widget inheriting from ModelSelect2MultipleWidget or HeavySelect2Mixin
3- Try to render the widget in a form

Expected Behavior

Widget should render without errors, as it did in v8.4.0