fix `_get_flats_weights`

tomoscan/scanbase.py

@@ -571,6 +571,8 @@ class TomoScanBase:
 
     def _get_flats_weights(self):
         """compute flats indexes to use and weights for each projection"""
+        if self.normed_flats is None:
+            return None
         flats_indexes = sorted(self.normed_flats.keys())
 
         def get_weights(proj_index):