Warn when get_graph tries to draw edge that doesn't exist (#4021)

libs/langgraph/langgraph/graph/graph.py

@@ -499,6 +499,11 @@ class CompiledGraph(Pregel):
         ) -> None:
             if end == END and END not in end_nodes:
                 end_nodes[END] = graph.add_node(self.get_output_schema(config), END)
+            if start not in start_nodes or end not in end_nodes:
+                logger.warning(
+                    f"Could not add edge from '{start}' to '{end}' due to missing nodes"
+                )
+                return
             return graph.add_edge(
                 start_nodes[start],
                 end_nodes[end],