我一直试图模拟我的TopicManagementResponse来测试我的测试类，但即使在使用TopicManagementResponse resposne = Mockito.mock(TopicManagementResponse.class)之后，它仍然说resposne是null。
这是模拟这个类的正确方法吗，或者有什么方法可以做到这一点？
我期望使用TopicManagementResponse对象测试类我的类。但它是null
我的班级：

@PostMapping("/subscribeToTopic")
    public int createTopic(@RequestBody Topic topic) throws IOException, FirebaseMessagingException {
 
        String platformID = topic.getPlatformId();
        //my internal method to get firebase messaging app
        FirebaseMessaging firebaseMessaging = fcmInitializer.getFirebaseMessaging(platformID);
 
        // Get the topic name from the "topic" field in the JSON
        String topicName = topic.getTopicName();
 
        // Check if the topic name is not null or empty
        if (topicName == null || topicName.isEmpty()) {
            return 0;
        }
        TopicManagementResponse response = firebaseMessaging.subscribeToTopic(topic.getToken(), topicName);
        return response.getSuccessCount();
 
    }

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我的测试类：

public void createTopic(){
FirebaseMessaging firebaseMessaging = mock(FirebaseMessaging.class);
    TopicManagementResponse response = mock(TopicManagementResponse.class);
    Topic topic = new Topic();
    topic.setToken("some token");
    topic.setTopic("some topic");
    
    when(firebaseMessaging.subscribeToTopic(topic.getToken(), topic.getTopic())).thenReturn(response);
    assertEquals(response.getSuccessCount, 1);
}

型