Null pointer exception unirest power mockito

I want to test this unirest method . I'am using powerMockito .

        try {
            HttpResponse<JsonNode> resp = Unirest
                    .post(url)
                    .header("Content-Type", "application/json")
                    .body(payload)
                    .asJson();
            if(resp.getStatus()!=200) {
                JSONObject error=new JSONObject();
                error.put("error", resp.getBody());
                error.put("status code from promote",resp.getStatus());
                return error;
            }
            return resp.getBody().getObject();
        } catch (UnirestException e) {
            e.printStackTrace();
        }

The problem is when I try to test it asJson() returns nullPointerException. This is my test .

@Before
    public void setUp() throws UnirestException, UnsupportedEncodingException {
        JSONObject payload=payloadForDockerPromote("","","","","","","","");
        JSONObject test=new JSONObject();

        JsonNode jsonNode = mock(JsonNode.class);
        JSONObject jsonObject = mock(JSONObject.class);
        PowerMockito.mockStatic(Unirest.class);   
        ReflectionTestUtils.setField(groupService, "baseServicesUrl", "http://foo");  
        when(Unirest.post("http://foo/stage/promotedocker")).thenReturn(requestWithBody);
        when(requestWithBody.header("Content-Type", "application/json")).thenReturn(requestWithBody);
        when(requestWithBody.body(payload)).thenReturn(requestBodyEntity);
        when(requestBodyEntity.asJson()).thenReturn(httpResponse);
        when(httpResponse.getBody()).thenReturn(jsonNode);
        when(jsonNode.getObject()).thenReturn(jsonObject);
        when(httpResponse.getStatus()).thenReturn(Integer.valueOf(200));
    }
    @Test
    public void groupPromoteDockerTest() throws Exception{
        GroupService myClass = Mockito.spy(new GroupService());
       JSONObject m=myClass.promoteDockerImage("","","","","","","","");
        assertEquals(m,null);
    }

Have you ever faced the same problem ?