Change color of Rectangle in a cell in DataGrid WPF VB.NET

[Sherlock_Holmes]

Hi people.
I have a DataGrid in WPF.
There are two columns in it.
All datas at the first column are a rectangle (I mean as Shape).
There is a button in the window, that when user hits the button, it fills the Rectangle of row 35, to Red.

But I don't know why VS gives me exception that it is nothing!

Would you please kindly help me on this?




What I have tried:

<Window x:Class="MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:RectangleREF"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <DataGrid x:Name="DGEditing"
                  HorizontalAlignment="Stretch" 
                  VerticalAlignment="Stretch"
                  HorizontalContentAlignment="Stretch"
                  VerticalContentAlignment="Stretch"
                  AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTemplateColumn Header="" IsReadOnly="True" Width="2*">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <Rectangle RadiusX="5" RadiusY="3.5" Fill="Black">
                            </Rectangle>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
                <DataGridTextColumn Header="Character" IsReadOnly="False" Width="22*" Binding="{Binding Path=CharacterName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
            </DataGrid.Columns>
        </DataGrid>
        <Button Content="Button" HorizontalAlignment="Left" Margin="650,149,0,0" VerticalAlignment="Top" Height="91" Width="91" Click="Button_Click"/>
    </Grid>
</Window>

Class MainWindow
    Public Class TemporaryItemsClass
        Public Property Rct As Boolean
        Public Property CharacterName As String
    End Class
    Public TemporaryItems As New System.Collections.ObjectModel.ObservableCollection(Of TemporaryItemsClass)
    Sub New()
        InitializeComponent()
        For I As Integer = 1 To 1000
            TemporaryItems.Add(New TemporaryItemsClass With {
                .Rct = False,
                .CharacterName = "Number" & I
                })
        Next I
        DGEditing.ItemsSource = TemporaryItems
    End Sub
    Private Sub Button_Click(sender As Object, e As RoutedEventArgs)
        Dim row = TryCast(DGEditing.ItemContainerGenerator.ContainerFromIndex(35), DataGridRow)
        Dim cell As DataGridCell = GetCell(DGEditing, row, 0)
        Dim rectangle = TryCast(cell.Content, Rectangle)
        rectangle.Fill = Brushes.Red
    End Sub
    Private Function GetCell(dg As DataGrid, rowContainer As DataGridRow, column As Integer) As DataGridCell
        Dim returner
        If rowContainer IsNot Nothing Then
            Dim presenter = VisualTreeHelper.GetParent(rowContainer)
            Dim cell As DataGridCell = TryCast(dg.Columns(column).GetCellContent(presenter), DataGridCell)
            If cell IsNot Nothing Then returner = cell
        End If
        Return returner
    End Function
End Class