RelayCommand works in C# but not in VB if not created in the constructor of the ViewModel

Question

I tried to implement button click command using mvvmlight relaycommand as described on this msdn page. Basically first I implemented in VB and C# as below. As you can see codes are almost same but button click doesnt work in VB unless I create the RelayCommand in the constructor in VB and raise the PropertyChanged event in the setter of the IsInProgress property (Commented out part). Can somebody explain me this difference between VB and C#? I added a link to the test project as well. test project

XAML: (same for VB and C#)

 <Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="50" />   

    </Grid.RowDefinitions>
    <ComboBox  Grid.Column="0" Grid.Row="0"   Height="20" Margin="20,50,20,0" ItemsSource="{Binding Path=Items}"  DisplayMemberPath="Name"
          IsSynchronizedWithCurrentItem="True"
              SelectedItem="{Binding SelectedItem}" />
    <Button  Grid.Column="0" Grid.Row="1" Content="Update" Height="20" Margin="20,0" VerticalAlignment="Center" Command="{Binding UpdateClick}"  />

</Grid>

VB view Model:

Public Class MainViewModel
    Inherits NotifyUIBase

    Private _IsInProgress As Boolean

    Public Property IsInProgress() As Boolean
        Get
            Return _IsInProgress
        End Get
        Set(ByVal value As Boolean)
            _IsInProgress = value

            'RaisePropertyChanged("IsInProgress")

        End Set
    End Property

    Private _Items As List(Of Item)
    Public Property Items() As List(Of Item)
        Get
            Return _Items
        End Get
        Set(ByVal value As List(Of Item))
            _Items = value
        End Set
    End Property

    Private _selectedItem As Item
    Public Property SelectedItem() As Item
        Get
            Return _selectedItem
        End Get
        Set(ByVal value As Item)
            _selectedItem = value
        End Set
    End Property


    Public Sub New()
        _IsInProgress = False
        GetData()

        '_UpdateClick = New RelayCommand(Async Sub()
        '                                    If IsInProgress = True Then
        '                                        Exit Sub
        '                                    End If

        '                                    IsInProgress = True
        '                                    UpdateClick.RaiseCanExecuteChanged()
        '                                    Await Update(False)

        '                                    IsInProgress = False
        '                                    UpdateClick.RaiseCanExecuteChanged()
        '                                End Sub, Function() Not IsInProgress)
    End Sub

    Private _UpdateClick As RelayCommand
    Public ReadOnly Property UpdateClick() As RelayCommand
        Get


            Return If(_UpdateClick Is Nothing, New RelayCommand(Async Sub()
                                                                    If IsInProgress = True Then
                                                                        Exit Sub
                                                                    End If

                                                                    IsInProgress = True
                                                                    UpdateClick.RaiseCanExecuteChanged()
                                                                    Await Update(False)

                                                                    IsInProgress = False
                                                                    UpdateClick.RaiseCanExecuteChanged()
                                                                End Sub, Function() Not IsInProgress), _UpdateClick)


        End Get

    End Property


    Private Sub GetData()
        _Items = New List(Of Item)
        _Items.Add(New Item With {.Name = "item1", .Value = "0"})
        _Items.Add(New Item With {.Name = "item2", .Value = "1"})
        RaisePropertyChanged("Items")
    End Sub


    Private Function Update(v As Boolean) As Task(Of Boolean)
        Throw New NotImplementedException()
    End Function
End Class

C# view Model

    namespace testWPF
{
   public  class MainViewModel :NotifyUIBase
    {
        public bool IsinProgress { get; set; }
        public MainViewModel()
        {
            IsinProgress = false;
            GetData();
        }

        public List<Item> Items { get; set; }

        public Item SelectedItem { get; set; }

        private RelayCommand  _UpdateClick;

        public  RelayCommand UpdateClick
        {

            get
            {

                return _UpdateClick
                  ?? (_UpdateClick = new RelayCommand(
                    async () =>
                    {
                        if (IsinProgress )
                        {
                            return;
                        }

                        IsinProgress  = true;
                        UpdateClick.RaiseCanExecuteChanged();

                        await Update();

                        IsinProgress  = false;
                        UpdateClick.RaiseCanExecuteChanged();
                    },
                    () => !IsinProgress ));
            }

        }

        private Task<bool> Update()
        {
            throw new NotImplementedException();
        }

        private void GetData()
        {
            Items = new List<Item>();
            Items.Add(new Item
            {
                Name = "item1",
                Value = "0"
            });
            Items.Add(new Item
            {
                Name = "item2",
                Value = "1"
            });
            RaisePropertyChanged("Items");
        }

    }
}

Code behind of the XAML:

C#

 public MainWindow()
 {
    InitializeComponent();
     this.DataContext = new  MainViewModel();
 }

VB

Public Class MainWindow

    Public Sub New()


        InitializeComponent()
        Me.DataContext = New MainViewModel()


    End Sub

End Class