system was nullptr Allegro5

#include <windows.h>
#include <iostream>
#include <conio.h>
#include <string>
#include <allegro5/allegro.h>
#include <allegro5/allegro_font.h>
#include "class.h"

//Zmienne globalne
int windowWidth = 640;
int windowHeight = 480;

int main(int argc, char **argv)
{
//inicjalizacja
game towerdefense;
towerdefense.init();

while (1) {
al_clear_to_color(al_map_rgb(0, 0, 0));
al_draw_text(towerdefense.font, al_map_rgb(255, 255, 255), 400, 300, ALLEGRO_ALIGN_CENTER, "Welcome to Allegro!");
al_flip_display();
}
al_rest(5.0);
al_destroy_display(towerdefense.display);
al_destroy_font(towerdefense.font);
return 0;
}

class wieza {
public:

std::string name;
int position;
int type;
int hp;
int dmg;

wieza();
};

class wrog {
public:

int position;
int typ;
int speed;
int hp;

wrog();
};

class game {
public:

void init() {
al_init();
al_init_font_addon();
}

ALLEGRO_DISPLAY* display = al_create_display(800, 600);
ALLEGRO_FONT* font = al_create_builtin_font();


};