How could I mock this and make sure to not get back null?

I have the following class that I want to unit test:

@RequiredArgsConstructor
@Service
public class Service {
    private final WebClient.Builder builder;
    private WebClient webClient;

    @PostConstruct
    public void init() {
        searchUri = "/search-uri";
    
        webClient = builder.baseUrl(searchUri)
            .defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
            .build();
    }    


    public ResponseSpec search() {
        
        return webClient
          .get()
          .uri(uriBuilder ->
              uriBuilder
                  .path("/search-uri")
                  // alot of query param, not important
                  .build()
          )
          .accept(MediaType.APPLICATION_JSON)
          .acceptCharset(StandardCharsets.UTF_8)
          .retrieve();
    }
}

This is my test class:

@ExtendWith(MockitoExtension.class)
@RunWith(JUnitPlatform.class)
public class ServiceTest {

@InjectMocks
private Service service;

@Mock
private WebClient webClient;

@Mock
private Builder builder;

@Test
public void testSearch() {
    
        when(builder.baseUrl(Mockito.anyString())).thenReturn(builder);
        when(builder.defaultHeader(Mockito.anyString(), Mockito.anyString())).thenReturn(builder);
        when(builder.build()).thenReturn(webClient); 
        issuerServiceImpl.init();
        
        WebClient.RequestHeadersUriSpec uriSpecMock = mock(WebClient.RequestHeadersUriSpec.class);
        WebClient.RequestHeadersSpec headersSpecMock = mock(WebClient.RequestHeadersSpec.class);
        WebClient.ResponseSpec responseSpecMock = mock(WebClient.ResponseSpec.class);
        
        
        when(webClient.get()).thenReturn(uriSpecMock);
        lenient().when(uriSpecMock.uri(Mockito.any(URI.class))).thenReturn(headersSpecMock);       
        
        issuerServiceImpl.searchIssuers("");
}
}

An exception (NullPointerException) happens in the line of .accept(MediaType.APPLICATION_JSON) because the uri() returns null and the code tries to call method .accept() on a null object.

I am not let to change the Service class. I can change only my test class. Not sure how could I make it to work.