用户控件作为组件.无法访问父窗口

我正在学习WPF和MVVM模式，我可能实现的东西不正确。

我想构建组件为我的应用程序的窗口使用用户控制对象，其中组件可以跨windows使用。例如，我有一个NewUnitUserControl，我打算将其用作对话框窗口的唯一组件，同时也用作主窗口的组件。

NewUnitUserControl.xaml:

<UserControl x:Class="Sample.Views.UserControls.NewUnitUserControl"
    ...                 
    xmlns:local="clr-namespace:Sample.Views.UserControls">
    <Grid>
        <StackPanel Orientation="Vertical">
            <Border>
                <StackPanel Orientation="Horizontal">
                    <Label>Name:</Label>
                    <TextBox Text="{Binding Unit.Name}" Width="136"/>
                </StackPanel>
            </Border>
            <Border HorizontalAlignment="Center">
                <StackPanel Orientation="Horizontal">
                    <Border>
                        <Button Command="{Binding CreateUnitCommand}">Create</Button>
                    </Border>
                    <Border>
                        <Button Command="{Binding CancelCommand}">Cancel</Button>
                    </Border>
                </StackPanel>
            </Border>
        </StackPanel>
    </Grid>
</UserControl>

NewUnitUserControl.xaml.cs:

namespace Sample.Views.UserControls
{
    public partial class NewUnitUserControl : UserControl
    {
        public NewUnitUserControl()
        {
            InitializeComponent();
            NewUnitViewModel nuvm = new NewUnitViewModel();
            DataContext = nuvm;
            if (nuvm.CloseAction == null)
            {
                var window = Window.GetWindow(this); // window evaluates to null 
                                                     // after this line.
                nuvm.CloseAction = new Action(window.Close);
            }
        }
    }
}

NewUnitViewModel.cs

namespace Sample.ViewModels
{
    internal class NewUnitViewModel : INotifyPropertyChanged
    {
        //Properties
        private Unit _unit;
        public Unit Unit
        {
            get { return _unit; }
            set { _unit = value; OnPropertyChanged("Unit"); }
        }
        public Action CloseAction { get; set; }
        private ICommand _createUnitCommand;
        public ICommand CreateUnitCommand
        {
            get
            {
                if (_unitUpdateCommand == null)
                    _unitUpdateCommand = new RelayCommand(param => CreateUnit(), param => true);
                return _unitUpdateCommand;
            }
        }
        private ICommand _cancelCommand;
        public ICommand CancelCommand
        {
            get
            {
                if(_cancelCommand == null)
                    _cancelCommand = new RelayCommand(param => Cancel(), param => true);
                return _cancelCommand;
            }
        }
        //Constructor
        public CreateUnitViewModel()
        {
            _unit = new Models.Unit();
        }
        //Methods
        public void CreateUnit()
        {
            Debug.Assert(false, String.Format("{0} was created.", Unit.Name));
        }
        public void Cancel()
        {
            this.CloseAction();
        }
        #region INotifyProperty Members
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(String propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion
    }
}

NewUnitDialogWindow.xaml:

<Window x:Class="Sample.Views.NewUnitWindow"
        ...
        xmlns:local="clr-namespace:Sample.Views"
        xmlns:controls="clr-namespace:Sample.Views.UserControls"
        >
    <controls:NewUnitUserControl />
</Window>

NewUnitDialogWindow.xaml.cs

namespace Sample.Views
{
    public partial class NewUnitWindow : Window
    {
        public NewUnitWindow()
        {
            InitializeComponent();
        }    
    }
}

完整源代码:GitHub

我立即面临的问题是，在我的实现中，我无法使用这里的解决方案访问用户控件的父窗口(参见NewUnitUserControl.xaml.cs中的评论)。我希望我的问题的根源是我没有正确理解MVVM模式。