WPF - 错误：[xxx]“不包含[zzz]的定义，并且找不到扩展方法[zzz]

I get an error while compiling:

编译时出错:

'SimulatorUi.MainWindow' does not contain a definition for 'UserCtrlSimulator' and no extension method 'UserCtrlSimulator' accepting a first argument of type 'SimulatorUi.MainWindow' could be found (are you missing a using directive or an assembly reference?) 'Sim