fix add functions to agent(#4937) (#5070)

fix bug: in class AgentBuilder~build() add function（list_of_functions to
agent，the return agent name is ‘None’，and the app crashed

## Related issue number

#4937

Author: davidxwwang

autogen/agentchat/contrib/agent_builder.py

@@ -182,7 +182,7 @@ class AgentBuilder:
     {agent_details}
 
     Hint:
-    # Only respond with the name of the agent that is most suited to execute the function and nothing else.
+    # Only respond with the name of the agent that is most suited to execute the function and nothing else.if there is no agent that is most suited to execute the function, respond with exactly "None"
     """
 
     UPDATED_AGENT_SYSTEM_MESSAGE = """
@@ -730,7 +730,7 @@ def _build_agents(
 
             if list_of_functions:
                 for func in list_of_functions:
-                    resp = (
+                    perferred_agent_name = (
                         self.builder_model.create(
                             messages=[
                                 {
@@ -747,28 +747,30 @@ def _build_agents(
                         .choices[0]
                         .message.content
                     )
+                    if perferred_agent_name in self.agent_procs_assign.keys():
+                        autogen.agentchat.register_function(
+                            func["function"],
+                            caller=self.agent_procs_assign[perferred_agent_name][0],
+                            executor=agent_list[0],
+                            name=func["name"],
+                            description=func["description"],
+                        )
 
-                    autogen.agentchat.register_function(
-                        func["function"],
-                        caller=self.agent_procs_assign[resp][0],
-                        executor=agent_list[0],
-                        name=func["name"],
-                        description=func["description"],
-                    )
-
-                    agents_current_system_message = [
-                        agent["system_message"] for agent in agent_configs if agent["name"] == resp
-                    ][0]
+                        agents_current_system_message = [
+                            agent["system_message"] for agent in agent_configs if agent["name"] == perferred_agent_name
+                        ][0]
 
-                    self.agent_procs_assign[resp][0].update_system_message(
-                        self.UPDATED_AGENT_SYSTEM_MESSAGE.format(
-                            agent_system_message=agents_current_system_message,
-                            function_name=func["name"],
-                            function_description=func["description"],
+                        self.agent_procs_assign[perferred_agent_name][0].update_system_message(
+                            self.UPDATED_AGENT_SYSTEM_MESSAGE.format(
+                                agent_system_message=agents_current_system_message,
+                                function_name=func["name"],
+                                function_description=func["description"],
+                            )
                         )
-                    )
 
-                    print(f"Function {func['name']} is registered to agent {resp}.")
+                        print(f"Function {func['name']} is registered to agent {perferred_agent_name}.")
+                    else:
+                        logger.debug(f"Function {func['name']} is not registered to any agent.")
 
         return agent_list, self.cached_configs.copy()