改造中的错误回应

Question

我想注册API＆＃34; http://chattymac.herokuapp.com/v1/account/register＆＃34;在android中使用改造。每当新用户注册时，我们都会得到响应，因为html字符串＆＃34;已成功注册＆＃34;但是当现有用户注册时，我们得到json格式的响应，app崩溃，给出NULLPointerException。我的代码如下：

    public interface APIService {


@Headers({"Content-Type: application/json",
"Authorization: Basic Z2VldGFuamFsaTpnZWV0YW5qYWxp"})
@POST("account/register")
Call<ResponseBody> createUser(@Body User user);

}

    public class User {
private String email;
private String password;

public User(String email, String password) {
    this.email = email;
    this.password = password;
}

public String getEmail() {
    return email;
}

public String getPassword() {
    return password;
}

}

    private void userSignUp() {
    //defining a progress dialog to show while signing up
    final ProgressDialog progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Signing Up...");
    progressDialog.show();

    String email = signUpEmail.getText().toString().trim();
    String password = signUpPassword.getText().toString().trim();

    //building retrofit object
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(APIUrl.BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();

    //Defining retrofit api service
    APIService service = retrofit.create(APIService.class);

    //Defining the user object as we need to pass it with the call
    User user = new User(email, password);

    //defining the call
    Call<ResponseBody> call = service.createUser(user);

    //calling the api
    call.enqueue(new Callback<ResponseBody>() {
        @Override
        public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
            //hiding progress dialog
            progressDialog.dismiss();

            try {
                 responseMessage = response.body().string();
            } catch (IOException e) {
                e.printStackTrace();
            }
            //displaying the message from the response as toast
            Toast.makeText(getApplicationContext(), responseMessage, Toast.LENGTH_LONG).show();
        }

        @Override
        public void onFailure(Call<ResponseBody> call, Throwable t) {
            progressDialog.dismiss();
            Toast.makeText(getApplicationContext(), t.getMessage(), Toast.LENGTH_LONG).show();
        }
    });
}

由于我是android的新手，如果有人可以帮我解决如何获得字符串和json响应。提前谢谢。