问运行时错误:用于POST请求的无效内存地址或零指针取消引用(Golang)

EN

//main.go
package main

import (
    "TestGoProject/src/Mappings"
    "TestGoProject/src/Models"
    _ "github.com/gin-gonic/gin"
    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/mysql"
    _ "net/http"
)

var db *gorm.DB

func main()  {
    initDb()
    Mappings.InitializeRoutes()

}

func initDb() {
    var err error
    db, err = gorm.Open("mysql", "root:helloworld@/testapp2?charset=utf8&parseTime=True&loc=Local")

    if err != nil {
        panic("failed to connect database")
    }
    db.AutoMigrate(&Models.BookModel{})
}

//router.go
package Mappings

import (
    "TestGoProject/src/Controllers"
    "github.com/gin-gonic/gin"
)

func InitializeRoutes()  {
    router := gin.Default()
    router.GET("/", Controllers.Index)
    router.POST("/createBook", Controllers.Create)
    _ = router.Run()
}

//BookController.go
package Controllers

import (
    "TestGoProject/src/Models"
    "github.com/gin-gonic/gin"
    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/mysql"
    "net/http"
)

var db = gorm.DB{}

func Index(c *gin.Context)  {
    c.JSON(http.StatusOK, gin.H{"status": http.StatusOK, "message": "Im here!"})
}

func Create(c *gin.Context)  {
    book := Models.BookModel{
        Title: c.PostForm("title"),
        Author: c.PostForm("author"),
    }

    db.Create(&book)
    c.JSON(http.StatusCreated, gin.H{"status": http.StatusCreated, "message": "Book created successfully!", "bookId": book.ID})
}

//bookModel.go
package Models

import "github.com/jinzhu/gorm"

type BookModel struct {
    gorm.Model
    Title string `json:"title"`
    Author string `json:"author"`
}

2020/05/25 10:54:50 [Recovery] 2020/05/25 - 10:54:50 panic recovered:
POST /createBook HTTP/1.1
Host: localhost:8080
Accept: */*
Accept-Encoding: gzip, deflate, br
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 48
Content-Type: application/json
User-Agent: PostmanRuntime/7.25.0


runtime error: invalid memory address or nil pointer dereference
/usr/local/go/src/runtime/panic.go:513 (0x102bd98)
        gopanic: reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))

[GIN] 2020/05/25-10:54:50|500|2.968348ms|::1|POST "/createBook"