我是 golang 和 gorm 框架的新手。這是我使用 gorm 獲取數據庫的代碼。數據庫Gopackage dbimport (    "fmt"    "github.com/jinzhu/gorm"    _ "github.com/lib/pq"    "github.com/teamhide/hfive_go/users"    "log")const (    DbHost     = "localhost"    DbPort     = "5432"    DbUser     = "1"    DbPassword = "1"    DbName     = "1")var db *gorm.DBvar err errorfunc Init() {    db, err = gorm.Open(        "postgres",        fmt.Sprintf(            "host=%s port=%s user=%s dbname=%s password=%s sslmode=disable",            DbHost,            DbPort,            DbUser,            DbName,            DbPassword,        ),    )    if err != nil {        log.Fatal(err.Error())    }    db.AutoMigrate(&users.User{})    defer db.Close()}func GetDB() *gorm.DB {    return db}用戶/usecase.gopackage usersimport (    "fmt"    "github.com/teamhide/gin_boilerplate/db")type UserUsecase interface {    RegisterUserUsecase(email, password1, password2 string) (bool, error)}func RegisterUserUsecase(email, password1, password2 string) (bool, error) {    db := db.GetDB()    fmt.Println(db)    return true, nil}在用例中，如您所知，我只是打印了 db 變量來檢查它是否具有正確的數據庫數據。但是當我執行它時，它會拋出錯誤。...runtime error: invalid memory address or nil pointer dereference.../Users/hide/go/src/github.com/teamhide/hfive_go/users/controllers.go:18 (0x15b3301)        UserController.RegisterDefaultUser: uc.RegisterUserUsecase(form.Email, form.Password1, form.Password2)...我的代碼有問題嗎？