fix unbound case

Author: tayheau

src/spikeinterface/widgets/rasters.py

@@ -446,6 +446,8 @@ def __init__(
                 depth_dict = {b: i for i, b in enumerate(unit_ids[s_args_depths].tolist())}
         elif sort_by_depth:
             raise AttributeError("`sort_by_depth=True` requires a SortingAnalyzer")
+        else:
+            depth_dict = None
 
         # Create dict of dicts structure
         spike_train_data = {}