使用Stripe为已有客户添加新卡片时遭遇NullPointerException问题求助

我最近在做Stripe相关的开发，想用测试令牌来指定要添加的卡片类型，测试令牌的名称是从Stripe官方文档里获取的。

我写了下面这段Java代码：

import com.stripe.Stripe;
import com.stripe.exception.StripeException;
import com.stripe.model.Card;
import com.stripe.model.Customer;
import java.util.HashMap;
import java.util.Map;

public class StripeExample {
    public static void main(String[] args) throws StripeException {
        final String API_KEY = System.getenv("STRIPE_API_KEY"); 
        assert API_KEY != null : "API_KEY SHOULD NOT BE NULL";

        Stripe.apiKey = API_KEY;

        // Retrieve the existing customer by ID
        Customer customer = Customer.retrieve(customerID);

        // Use a Stripe test token instead of raw card details
        String testToken = "tok_visa"; // Simulates a Visa card

        // Attach the test card to the customer
        Map<String, Object> sourceParams = new HashMap<>();
        sourceParams.put("source", testToken);

        Card card = (Card) customer.getSources().create(sourceParams);
        System.out.println("Card added successfully: " + card.getId());
    }
}

但运行后得到了下面的错误：

null
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.stripe.model.PaymentSourceCollection.create(java.util.Map)" because the return value of "com.stripe.model.Customer.getSources()" is null
    at org.example.Application.main(Application.java:82)

我本来想给客户成功添加卡片并输出卡片ID，结果却遇到了空指针异常，提示customer.getSources()的返回值是null，没法调用create方法。有没有朋友能帮我看看这是怎么回事，该怎么解决呀？

备注：内容来源于stack exchange，提问作者Marius Carchilan