package main

import (
	"encoding/json"
	"fmt"
)

type W2 interface {
	json.Marshaler
	json.Unmarshaler
}

type Data struct {
	ID string `json:"id"`
	W2 `json:"-"`
}

func main() {
	var d Data
	marshal, err := json.Marshal(d)
	if err != nil {
		return
	}
	fmt.Println(string(marshal))
}

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x49471b]