message: [bugfix] only pop slice_id when it exists in url
diff: @@ -1677,6 +1677,7 @@ class Superset(BaseSupersetView):
  datasource_id = args.get('datasource_id')
  if action in ('saveas'):
  + if 'slice_id' in d:
  d.pop('slice_id') # don't save old slice_id
  slc = models.Slice(owners=[g.user] if g.user else [])
  