我是新來的，試圖創(chuàng)建一個登錄函數(shù)，當(dāng)試圖從數(shù)據(jù)庫中查詢行時，我收到了這個錯誤：runtime error: invalid memory address or nil pointer dereference此行引起的崩潰：    result := db.QueryRow("SELECT password FROM users WHERE email=$1", credentials.Email)這是代碼：    type Credentials struct {    Email    string `json:"email", db:"email"`    Password string `json:"password", db:"password"`}func SignIn(w http.ResponseWriter, r *http.Request) {    credentials := &Credentials{}    err := json.NewDecoder(r.Body).Decode(credentials)    if err != nil {        // If there is something wrong with the request body, return a 400 status        w.WriteHeader(http.StatusBadRequest)        return    }    psqlInfo := fmt.Sprintf("host=%s port=%d user=%s "+"dbname=%s sslmode=disable", dbInfo.Host, dbInfo.Port, dbInfo.User, dbInfo.DBname)    db, err := sql.Open("postgres", psqlInfo)    println(credentials.Email)    result := db.QueryRow("SELECT password FROM users WHERE email=$1", credentials.Email)    defer db.Close()    if err != nil {        //If there is an issue with the database, return a 500 error.        w.WriteHeader(http.StatusInternalServerError)        return    }    //We create another instance of 'Credentials' t store the credentials we get from the database    storedCreds := &Credentials{}    // Store the obtained password in `storedCreds`    err = result.Scan(&storedCreds.Password)    if err != nil {        // If an entry with the email does not exist, send an "Unauthorized"(401) status        if err == sql.ErrNoRows {            w.WriteHeader(http.StatusUnauthorized)            return        }        //If the error is of any other type, send a 500 status        w.WriteHeader(http.StatusInternalServerError)        return    }    if credentials.Password != storedCreds.Password {        //The two passwords does not match, return a 401 status        w.WriteHeader(http.StatusUnauthorized)    }}我檢查了credentials.Email 不為空，我不明白是什么導(dǎo)致了這個錯誤。