MAUI EventToCommandBehavior

问：

XAML：

<ContentPage.Behaviors>
    <toolkit:EventToCommandBehavior
        EventName="OnAppearing"
        Command="{Binding AppearingExeCommand}" />
</ContentPage.Behaviors>

视图模型：

[RelayCommand]
public void AppearingExe()
{
    try
    {
        WeakReferenceMessenger.Default.Reset();
        if (!WeakReferenceMessenger.Default.IsRegistered<MyMessage>(this))
        {
            WeakReferenceMessenger.Default.Register<MyMessage>(this, OnMessageReceived);
        }
    }
    catch(Exception er)
    {
        Shell.Current.DisplayAlert("Error!", er.Message, "OK");
    }
}

运行错误：调用目标已引发异常

原因是什么？我应该如何处理