public void ConfigureServices(IServiceCollection services)
{
     ...
    services.AddMvc();
    services.AddSession(); // After AddMvc()
    services.AddHttpContextAccessor();
     ...
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
     ...
    app.UseSession();  // Before UseMvc()
    app.UseMvc();
     ...
}