Testing with mockito in Springboot

I am working on unit tests to map an object to an another : this is my test class. The issue is a NPE caused by the first line in GeneratEvent where its supposed to get a randomId(it works if its hardcoded) the wierd thing is you can see that the idGenerator works fine while testing outside that class

public class MessageTest {
    
    @InjectMocks
    private IdGeneratorConfig idGenerator;

    @Mock
    private IKafkaService kafkaService;

   
    @Mock
    private IKafkaService kafkaService;

    @ExtendWith(MockitoExtension.class)
    @InjectMocks
    private IngestedEventGenerator ingestedEventGenerator;

    @Before
    public void setUp() {
        MockitoAnnotations.openMocks(this);
    }

    @Test
    public void testGenerateAndSendEventIngestion() {

        Message message = new Message();
        message.setMsgId(12356486);
        message.setCustomId("custom1");
        message.setRecipientEmail("exemple@example.com");
        message.setMetadata("metadatamail");

        long expectedEventId =idGenerator.newIdGenerator().ranId() ;
        System.out.println(expectedEventId); // works fine !!!

        MessageEvent messageEvent = (MessageEvent) messageEventGenerator.generateEvent(message);  // null pointer exception(Id null !!?)
        messageEvent.setEventId(expectedEventId);  

        assert messageEvent != null;
        assertEquals(12360055, messageEvent.getMsgId());
        assertEquals("custom1", messageEvent.getCustomId());
        assertEquals("exemple@example.com",         messageEvent.getRecipient());
        assertEquals("metadatamail", messageEvent.getMetadata());

        messageEventGenerator.sendEvent(messageEvent);

        verify(kafkaService, times(1)).sendEvent(messageEvent, "Eve");
    }

and this is the class that iam testing : i am mapping a message to an event (MesssageEvent extend Event)

public class MessageEventGenerator implements IEventGenerator {

    @Autowired
    private IdGeneratorConfig idGenerator;
    @Autowired
    private IKafkaService kafkaService;


    @Override
    public void sendEvent(Event event) {

        kafkaService.sendEvent(event,"newEvent");
    }

    @Override
    public Event generateEvent(Message message) {

        long Id =idGenerator.newIdGenerator().ranId() ; // null pointer exception
        MessageEvent messageEvent = new MessageEvent();
        messageEvent.setMsgId(message.getMsgId());
        messageEvent.setCustomId(message.getCustomId());
        messageEvent.setEventId(SnowId);
        messageEvent.setMetadata(message.getMetadata());

        return  messageEvent;

    }

        @Configuration
public class IdGeneratorConfig {


    @Bean
    @Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
    public IdGenerator idGenerator() {
        return new IdGenerator();
    }
}

You need to mock both the function that generates the ID and also the interface or the config that is calling the genetrated ID.