Write unit test in spring boot but get error because of converter class

I use converter class,I get null pointer exception error in unit test.
But I use return accountDto instead of converter class
it's running.
Code is running at the Postman.
Please give me some advise.

@Test
void store() {
    Date date = new Date();
    Account accountMock = mock(Account.class);
    AccountDto accountDto = new AccountDto();
    accountDto.setId(randomUUID);
    accountDto.setName("Test-Name");
    accountDto.setSurname("Test-Lastname");
    accountDto.setEmail("Test-Email");
    accountDto.setBirth_date(date);
    accountDto.setPassword("Test-Email");
    accountDto.setStatus(OPEN);
 
    when(accountMock.getId())
       .thenReturn(String.valueOf(randomUUID));
    when(accountRepository.save(ArgumentMatchers.any(Account.class)))
       .thenReturn(accountMock);
    
    AccountDto result = accountService.store(accountDto);
    
    assertEquals(result.getName(), accountDto.getName());
    assertEquals(result.getId(), String.valueOf(randomUUID));
}

Service Method =>

@Transactional
public AccountDto store(AccountDto accountDto) {
    Account account = new Account();
    account.setName(accountDto.getName());
    account.setSurname(accountDto.getSurname());
    account.setEmail(accountDto.getEmail());
    account.setBirth_date(accountDto.getBirth_date());
    account.setPassword(accountDto.getPassword());
    account.setStatus(accountDto.getStatus());
    final Account accountDb = repository.save(account);
    accountDto.setId(accountDb.getId());

    return converter.convertFromEntity(accountDb);
}

Converter class =>

 /**
 * Converts Entity to DTO.
 *
 * @param entity domain entity
 * @return The DTO representation - the result of the converting function application on domain
 * entity.
 */
public final T convertFromEntity(final U entity) {
    return fromEntity.apply(entity);
}

Error =>

java.lang.NullPointerException
	at com.kablanfatih.tddexample.converter.Converter.convertFromEntity(Converter.java:51)
	at com.kablanfatih.tddexample.service.impl.AccountServiceImpl.store(AccountServiceImpl.java:42)
	at com.kablanfatih.tddexample.service.impl.AccountServiceImplTest.store(AccountServiceImplTest.java:87)

You have two options in my opinion. First you can mock your Converter class or second you initialize your converter properly and use it in your test. I prefer the first solution because I can better control the behaviour of the Converter in the context of your unit test.

What I am getting here is that you are writing tests after writing code so you landed in this issue.

Recommended solution should be mocking converter class and control the response and test converter method separately. This is how TDD works.

    @Mock
    Converter converter;

    when(converter.yourMockedMthord).thenReturn(your_output);

And you can write ConverterTest.java class to test behaviour of method.