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

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:

Code:

---

<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>

---

Code:

---

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

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I am not sure what you are asking, however if you mean you're getting null-reference errors I see a potential issue in the code you provided. Your GetCell function looks like it could return "Nothing". You could fix that by setting the "Returner" variable to an empty DataGridCell or something. Also I see that the code calling GetCell isn't checking for whether "Nothing" is being returned.