Error while converting json to struct from Go

 func MakeMap(w http.ResponseWriter, r *http.Request) {
	// userInfo := context.Get(r, "userInfo").(model.User)
	type _getData struct {
		Title string   `json:"title"`
		Tag   []string `json:"tag"`
	}
	var getData _getData
	err := json.NewDecoder(r.Body).Decode(&getData)
	if err != nil {
		panic(err.Error())
	}
	fmt.Print(getData)

}

When I run the above code, I get the following error

2021/08/24 13:56:54 http: panic serving 127.0.0.1:50619: runtime error: invalid memory address or nil pointer dereference
goroutine 23 [running]:
net/http.(*conn).serve.func1(0x140001e9180)
        /usr/local/go/src/net/http/server.go:1824 +0x108
panic(0x10505b860, 0x10522f240)
        /usr/local/go/src/runtime/panic.go:971 +0x3f4
traveling/controller/mapController.MakeMap(0x1050b5630, 0x140001f40e0, 0x1400018aa00)
/Users/choeyunseog/traveling/traveling/controller/mapController/mapController.go:20 +0x3c

I've just started studying, I'm not sure why I'm having this problem, please help

err := json.NewDecoder(r.Body).Decode(&getData) 

I get the following error when i change code line 20 like above

 2021/08/24 14:16:44 http: panic serving 127.0.0.1:51396: invalid character '-' in numeric literal
goroutine 23 [running]:
net/http.(*conn).serve.func1(0x140001e9360)
        /usr/local/go/src/net/http/server.go:1824 +0x108
panic(0x100d85d00, 0x14000206070)
        /usr/local/go/src/runtime/panic.go:971 +0x3f4
traveling/controller/mapController.MakeMap(0x100df1630, 0x140001f40e0, 0x1400018aa00)
        /Users/choeyunseog/traveling/traveling/controller/mapController/mapController.go:24 +0x194
net/http.HandlerFunc.ServeHTTP(0x100de75d8, 0x100df1630, 0x140001f40e0, 0x1400018aa00)
        /usr/local/go/src/net/http/server.go:2069 +0x40

To get the multipart form data from a POST/PUT/PATCH request's body you can use the ParseMultipartForm method to parse the body and then access the data through the PostForm field. Or you can use FormValue to get just the first value associated with the form's field.

maxMemory := 32<<20
if err := r.ParseMultipartForm(maxMemory); err != nil {
    panic(err)
}

fmt.Println(_getData{
    Title: r.FormValue("title"), // FormValue returns string
    Tag:   r.PostForm["tag[]"],  // PostForm is a map of []string
})

You can use to parse form data into json like annotated struct using package github.com/senpathi/paramex. Struct fields must be annotated with param keyword and tag name is key of the form data.

your struct should be as below.

type _getData struct {
		Title string   `param:"title"`
		Tag   []string `param:"tag[]"`
	}

This is the updated MakeMap handler function for your postman request mentioned in the question

func MakeMap(w http.ResponseWriter, r *http.Request) {
	// userInfo := context.Get(r, "userInfo").(model.User)
	type _getData struct {
		Title string   `param:"title"`
		Tag   []string `param:"tag[]"`
	}

	// this needed because u send data from Postman as multipart/form-data
	maxMemory := 32<<20
	if err := r.ParseMultipartForm(int64(maxMemory)); err != nil {
		panic(err)
	}

	var getData _getData

	extractor := paramex.NewParamExtractor()
	err := extractor.ExtractForms(&getData, r)
	if err != nil {
		panic(err.Error())
	}

	fmt.Print(getData)
	//Output: {defaultMap [travelling travelling2]}
}