使用sync.WaitGroup panics完成的WebCrawl练习。我做错了什么？Go惯用解决方案是什么？

package main

import (
    "fmt"
    "net/http"
    "sync"
)

func crawl(url string, wg *sync.WaitGroup) {
    defer wg.Done() // 确保在函数返回前调用Done

    resp, err := http.Get(url)
    if err != nil {
        fmt.Println(err)
        return
    }
    defer resp.Body.Close()

    // 处理网页内容...
    fmt.Println("Crawled", url)
}

func main() {
    var wg sync.WaitGroup
    urls := []string{
        "http://example.com",
        "http://example.org",
        "http://example.net",
    }

    for _, url := range urls {
        wg.Add(1) // 在启动goroutine之前调用Add
        go crawl(url, &wg)
    }

    wg.Wait() // 等待所有goroutine完成
    fmt.Println("Crawling finished")
}