Mockito mock is giving null for given test cases

I am using the SpringRunner to run the Junit mockito test case , below is the class , i was trying to write the test case , but getting null object

    public class AccountManager {

        public String getToken() throws Exception {
               @Autowired
               RestClient restClient;

                String auth = apiUserPrefix + apiUserName + BatchJobConstants.COLON + apiUserPassword;
                byte[] encodedAuth = Base64.encodeBase64(
                        auth.getBytes(StandardCharsets.UTF_8));
                String authHeader = BatchJobConstants.BASIC_SPACE + new String(encodedAuth);
                String token= null;
                MultiValueMap<String, String> data = new LinkedMultiValueMap<>();
                data.add("grant_type", "client_credential");
                String accManagerUrl = accManagerHost+":"+accManagerPort+"/"+accManagerResPath;
                RestResponseObject responseObject = null;

                try {
                    responseObject = restClient.execute(accManagerUrl, HttpMethod.POST, data, String.class, authHeader);
                    if (responseObject != null && responseObject.getPayload() != null && responseObject.getStatusCode() == HttpStatus.OK) {
                        JsonElement responseJson = (JsonElement) responseObject.getPayload();
                        if (responseJson.isJsonObject()) {
                            token= responseJson.getAsJsonObject().get(BatchJobConstants.ACCESS_TOKEN).getAsString();
                        }catch(RunTimeException e) {
//e
}
    return token;
        }

//Junit test case

@RunWith(SpringRunner.class)
    public class AccountManagerTest {
    @InjectMocks
        AccountManager accountManager;
    @Mock
         RestClient restClient;
 @Test
    public void getTokenAccMgrSucess() throws Exception {
        RestResponseObject restResponseObject = Mockito.mock(RestResponseObject.class);

        Mockito.when(restClient.execute(Mockito.anyString(), Mockito.any(HttpMethod.class),
                Mockito.anyString(), Mockito.eq(String.class), Mockito.anyString())).thenReturn(restResponseObject);
        String token = accountManagerTokenProvider.getToken();
        Assert.assertEquals("Token value {} ", null, token);

    }

    }

But still the below code return null value even after mocking this, can you please help how to mock this.

responseObject = restClient.execute(accManagerUrl, HttpMethod.POST, data, String.class, authHeader);

Note: Only Mockito needs to use no powermockito

Finally worked with mockito only just user any() instead of anyString(), since the Object is not matching with string only

Mockito.when(restClient.execute(Mockito.any(), Mockito.any(HttpMethod.class),
                Mockito.any(), Mockito.eq(String.class), Mockito.any())).thenReturn(restResponseObject);