getAttributeGroup method Null safety

Implementation

AttributeGroup getAttributeGroup(final String attributeId) {
  for (final AttributeGroup attributeGroup in attributeGroups!) {
    for (final Attribute item in attributeGroup.attributes!) {
      if (item.id == attributeId) {
        return attributeGroup;
      }
    }
  }
  throw Exception('unknown attribute group for $attributeId');
}