我想为Azure函数创建集成测试。我尝试为安装Azure函数创建主机，但在Configure方法中出现NullReference异常。在集成项目上下文中执行Configure方法时，如何设置配置？

Startup.cs

public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
     {
            base.ConfigureAppConfiguration(builder);

            var configurationBuilder = builder.UseAppSettings()
                .ConfigurationBuilder.AddEnvironmentVariables().Build();
            var connectionString = configurationBuilder["appConfiguration"];

            
            // other setup for App Config
     }

     public override void Configure(IFunctionsHostBuilder builder)
     {
            _configuration = builder.GetContext().Configuration;
            // using that configuration below 
     }

IntegrationHost.cs

var startup = new MyNamespace.Functions.Startup();
 var host = new HostBuilder()               
          .ConfigureWebJobs((context, builder) =>
           {
               startup.Configure(builder);

           })
           .Build();