Update doubly_linked_list_two.py

Author: MaximSmolskiy

Diff for: data_structures/linked_list/doubly_linked_list_two.py

@@ -105,6 +105,8 @@ def insert_before_node(self, node: Node, node_to_insert: Node) -> None:
         node.previous = node_to_insert
 
     def insert_after_node(self, node: Node | None, node_to_insert: Node) -> None:
+        assert node is not None
+
         node_to_insert.previous = node
         node_to_insert.next = node.next