课程的一部分：

#include <SDL2/SDL.h>

class Application {
    static std::shared_ptr<SDL_Window> window;
}

定义：

#include "Application.h"
std::shared_ptr<SDL_Window> Application::window{};

bool Application::init() {  
    SDL_Window *window_ = nullptr;
    if((window_ = SDL_CreateWindow(title.c_str(),SDL_WINDOWPOS_UNDEFINED,window_width,window_height,NULL)
        ) == nullptr) {
        std::cerr << "creating window Failed: " << SDL_GetError() << std::endl;
    }

    window.reset(window_);
}

错误出现在’window.reset()’.是什么原因以及如何解决此问题？