Reservoirs: Fix loading without nodes.

src/Model/Reservoir/Reservoir.py

@@ -310,7 +310,7 @@ class Reservoir(SQLSubModel):
                 new_reservoir.set_as_deleted()
 
             new_reservoir._node = None
-            if node_id != -1:
+            if node_id != -1 and node_id is not None:
                 new_reservoir._node = next(
                     filter(
                         lambda n: n.id == node_id, data["nodes"]