This small PR should be enough to get rid of the deprecation warning:
wagtail_advanced_form_builder/models/abstract_advanced_form_mixin.py:212: RemovedInWagtail50Warning: wagtail.admin.edit_handlers.StreamFieldPanel is obsolete and should be replaced by wagtail.admin.panels.FieldPanel
This small PR should be enough to get rid of the deprecation warning: