nullreferenceexception:对象引用未设置为数据库调用中对象的实例

我在存储库类的一个方法中遇到了一个空引用异常,当我试图检索模型的所有记录时,我读过这个异常,我认为我理解它,但我不明白为什么它会在我的代码中发生。

我有这个模型:

public class Post
{

    [Key]
    public int Id { get; set; }
    public string Title { get; set; } = "";
    public string Body { get; set; } = "";
    public DateTime? Created { get; set; } = DateTime.Now;
}

这是我的背景:

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions options) : base(options) { }
    public DbSet<Post> Posts { get; set; }
}

我的存储库类中的方法“GetAllPost()”:

    public async Task<List<Post>> GetAllPost()
    {
        var listOfPost = _context.Posts;//here is where the exception is thrown
        return await listOfPost.ToListAsync();
    }

这是我的整个存储库类:

public class Repository : IRepository
{
    private readonly AppDbContext _context;

    public Repository(AppDbContext context) {
        _context = context;
    }

    public async Task AddPost(Post post)
    {
        await _context.Posts.AddAsync(post);
        await _context.SaveChangesAsync();
    }

    public async Task<List<Post>> GetAllPost()
    {
        var listOfPost = _context.Posts;

        return await listOfPost.ToListAsync();
    }

    public async Task<Post> GetPost(int id)
    {
        return await _context.Posts.FirstOrDefaultAsync(x=> x.Id == id);
    }

    public async Task RemovePost(Post post)
    {
         _context.Remove(post);
         await _context.SaveChangesAsync();
    }

    public async Task UpdatePost(Post post)
    {
         _context.Posts.Update(post);
        await _context.SaveChangesAsync();
    }
}

这是我的ConfigureService方法(顺便说一句,我使用的是postgres):

public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();

           services.AddEntityFrameworkNpgsql().AddDbContext<AppDbContext>(
               x => x.UseNpgsql(
                   Configuration.GetConnectionString("DefaultConnection")
                   )
               );

            services.AddTransient<IRepository, Repository>();

        }

我已通过迁移创建并填充了数据库