fix regression

Author: dmarek-flex

tidy3d/components/microwave/path_spec_generator.py

@@ -1,5 +1,6 @@
 """Module for automatic determination of voltage and current integration specifications."""
 
+from itertools import chain
 from math import isclose
 
 import shapely
@@ -318,5 +319,5 @@ def _apply_symmetry(
                     PathSpecGenerator._apply_symmetry_to_box(box, dim, sim_center[dim])
                     for box in result
                 ]
-                result = result.extend(tmp_list)
+                result = list(chain.from_iterable(tmp_list))
         return result