Mock throwing a null pointer exception. New to mockito

I'm trying to test the following method

  MainClass {
    ....
        Client client;
        WebTarget target;
     boolean doLogin(MultivaluedMap<String, String> headers) {

             client =getRestClient();
             target = client.target(BASE_URL))
                      .path("v1/login");
              MultivaluedMap<String, Object> castedHeaders = castMap(headers);//casts headers by entry.
               Response loginRsp = target
                    .request().headers(castedHeaders).     
                   post(Entity.entity(buildIusLoginEntity(),
                            MediaType.APPLICATION_JSON_TYPE));
                if (loginRsp.getStatus() != HttpServletResponse.SC_OK) {

                    return false;
                 }
                 return true;
            }
    }

Using the following test class

@Test
  public void testdoLoginNegative() {
               MainClass m = spy(new MainClass());
            Client mockClient = mock(Client.class);
            WebTarget target = mock(WebTarget.class);

            Response loginRsp = Response.status(500).build();
            doReturn(mockClient).when(m).getRestClient();
    when(mockClient.target(anyString()).path(anyString())).thenReturn(target);
            when(target.request().headers(any(MultivaluedMap.class)).post(any(Entity.class))).thenReturn(loginRsp);// this line throws a Null pointer exception. 
            Assert.assertFalse(m.doIusLogin(getMockHeaders()));

     }

However, my mock seems to show a null pointer exception as indicated in the source code. Any thoughts on what I could be doing wrong.. would be greatly appreciated.