Webclient post exchange mockito unit test

Trying to write a mockito test for the below service code:

                .build()
                .post()
                .uri(fullURL +  queryPart )
                .contentType(MediaType.MULTIPART_FORM_DATA)
                .headers(headers -> headers.set("Authorization",token))
                .body(BodyInserters.fromMultipartData(parts))
                .exchange()
                .flatMap(clientResponse -> clientResponse.bodyToMono(String.class))
                .flux()
                .blockFirst();```


Mockito code fails for Java language null pointer exception at exchange (last line in the below code)..
```when(webClientBuilder.build()).thenReturn(webClient);
        when(webClient.post()).thenReturn(requestBodyUriSpec);
        when(requestBodyUriSpec.uri(anyString())).thenReturn(requestBodySpec);
        when(requestBodySpec.contentType(any(MediaType.class))).thenReturn(requestBodySpec);
        when(requestBodySpec.headers(any())).thenReturn(requestBodySpec);
        when(requestBodySpec.body(any())).thenReturn(requestHeadersSpec);
when(requestBodySpec.exchange().flatMap(any()).flux().blockFirst()).thenReturn(clientResponse.bodyToMono(String.class));

Any input will be appreciated..