fix STARBackend.drop_tips96

Author: rickwierenga

pylabrobot/liquid_handling/backends/hamilton/STAR_backend.py

@@ -2228,7 +2228,9 @@ async def drop_tips96(
     if isinstance(drop.resource, TipRack):
       tip_spot_a1 = drop.resource.get_item("A1")
       position = tip_spot_a1.get_absolute_location() + tip_spot_a1.center() + drop.offset
-      position.z = tip_spot_a1.parent.get_absolute_location().z + 1.45
+      tip_rack = tip_spot_a1.parent
+      assert tip_rack is not None
+      position.z = tip_rack.get_absolute_location().z + 1.45
       # This should be the case for all normal hamilton tip carriers + racks
       # In the future, we might want to make this more flexible
       assert abs(position.z - 216.4) < 1e-6, f"z position must be 216.4, got {position.z}"