为何在使用SDL_Rect时报错“**MainBackGroundRect** 是 nullptr？

#include<stdio.h>
#include"SDL.h"
#include"SDL_image.h"
#include"SDL_ttf.h"
#include<stdlib.h>
SDL_Rect *MainBackGroundRect ;
    MainBackGroundRect->x = 0;
    MainBackGroundRect->y = 0;
    MainBackGroundRect->w = MainBackGroundSurface->w;
    MainBackGroundRect->h = MainBackGroundSurface->h;