创建：运行时错误：无效的内存地址或nil指针取消引用

Question

我正在尝试使用Mux和GORM创建Go REST API，但是每次我尝试使用db.Create()向表中插入新行时，它都会弹出runtime error: invalid memory address or nil pointer dereference

代码如下：

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "net/http"
    "time"

    "github.com/gorilla/mux"

    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/postgres"
    uuid "github.com/satori/go.uuid"
)

type MetaResponse struct {
    Message string `json:"message"`
    Code    string `json:"code"`
}

type DataResponse interface {
}

type ResponseEnvelope struct {
    Meta MetaResponse `json:"meta"`
    Data DataResponse `json:"data"`
}

var db *gorm.DB

type Base struct {
    ID        uuid.UUID `gorm:"type:uuid;primary_key;" json:"id"`
    CreatedAt time.Time
    UpdatedAt time.Time
    DeletedAt *time.Time `sql:"index"`
}

// BeforeCreate will set a UUID rather than numeric ID.
func (base *Base) BeforeCreate(scope *gorm.Scope) error {
    uuid, err := uuid.NewV4()
    if err != nil {
        return err
    }
    return scope.SetColumn("ID", uuid)
}

type Merchant struct {
    Base
    MerchantPhoneNumber    string
    MerchantActivationType int
}

func InitializeMerchantHandler(w http.ResponseWriter, r *http.Request) {

    var resp ResponseEnvelope
    var merchant Merchant

    merchant = Merchant{
        MerchantPhoneNumber:    "SomeNumber",
        MerchantActivationType: 1,
    }

    db.Create(&merchant)

    resp = ResponseEnvelope{
        Meta: MetaResponse{
            Message: "Sukses",
            Code:    "2001",
        },
        Data: merchant,
    }
    json.NewEncoder(w).Encode(resp)
    return

}

func main() {

    db, err := gorm.Open("postgres", "host=localhost port=5432 user=postgres dbname=test password=password sslmode=disable")
    if err != nil {
        log.Panic(err.Error())
    }
    defer db.Close()
    db.AutoMigrate(&Merchant{})
    fmt.Println(db.DB().Ping()) // -> returns <nil>

    router := mux.NewRouter().StrictSlash(true)

    // Initialize Merchant Router
    router.HandleFunc("/merchant/init", InitializeMerchantHandler).Methods("POST")

    log.Fatal(http.ListenAndServe(":8080", router))

}

如何解决此问题？ 与Mux或GORM有关吗，因为如果我使用GORM网站中的代码片段对其进行测试，则db.Create()应该可以正常工作。

完全错误：

$ go run main.go
<nil>
2019/12/03 16:13:30 http: panic serving [::1]:58778: runtime error: invalid memory address or nil pointer dereference
goroutine 6 [running]:
net/http.(*conn).serve.func1(0xc000220000)
        c:/go/src/net/http/server.go:1767 +0x140
panic(0x813d20, 0xbb85c0)
        c:/go/src/runtime/panic.go:679 +0x1c0
github.com/jinzhu/gorm.(*DB).clone(0x0, 0xc0000542a0)
        C:/Users/Abhishta/go/src/github.com/jinzhu/gorm/main.go:848 +0x2d
github.com/jinzhu/gorm.(*DB).NewScope(0x0, 0x83e580, 0xc0000542a0, 0x409e26)
        C:/Users/Abhishta/go/src/github.com/jinzhu/gorm/main.go:201 +0x36
github.com/jinzhu/gorm.(*DB).Save(0x0, 0x83e580, 0xc0000542a0, 0xc0000542a0)
        C:/Users/Abhishta/go/src/github.com/jinzhu/gorm/main.go:466 +0x4a
main.InitializeMerchantHandler(0x921920, 0xc00022c000, 0xc00021e300)
        C:/Users/Abhishta/go/src/cash-it-pos-merchant-service/main.go:64 +0x130
net/http.HandlerFunc.ServeHTTP(0x8a73f0, 0x921920, 0xc00022c000, 0xc00021e300)
        c:/go/src/net/http/server.go:2007 +0x4b
github.com/gorilla/mux.(*Router).ServeHTTP(0xc000200000, 0x921920, 0xc00022c000, 0xc00021e100)
        C:/Users/Abhishta/go/src/github.com/gorilla/mux/mux.go:210 +0xe9
net/http.serverHandler.ServeHTTP(0xc00020e000, 0x921920, 0xc00022c000, 0xc00021e100)
        c:/go/src/net/http/server.go:2802 +0xab
net/http.(*conn).serve(0xc000220000, 0x9223e0, 0xc00001a180)
        c:/go/src/net/http/server.go:1890 +0x87c
created by net/http.(*Server).Serve
        c:/go/src/net/http/server.go:2927 +0x395