Need to focus textbox inside treeview item on selection changed

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WPF

C#7.0

Treeview has multiple Treeview items, each TreeviewItem has 3 textboxes and one button, when any treeview is selected we need to focus the first textbox inside it.Its changing in code but not updating the UI.

What I have tried:

Wpf code
<pre>  <HierarchicalDataTemplate ItemsSource="{Binding GroupMappings}" DataType="{x:Type models:GroupModel}" >
            <Border BorderBrush="Gray" BorderThickness="0,0,0,0" Margin="-5,0,0,0" >
                <StackPanel Orientation="Horizontal" Height="30"  KeyboardNavigation.TabNavigation="Cycle"
                    KeyboardNavigation.ControlTabNavigation="Continue" KeyboardNavigation.DirectionalNavigation="None">
                    <StackPanel.Style>
                        <Style TargetType="{x:Type StackPanel}">
                            <Setter Property="IsEnabled" Value="False" />
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding IsSelected}" Value="true">
                                    <Setter Property="IsEnabled" Value="True" />
                                    <Setter Property="Background" Value="PowderBlue" />
                                </DataTrigger>
                            </Style.Triggers>
                        </Style> 
                    </StackPanel.Style>
                    <TextBox Name="GroupText" Width="70" Text="{Binding MaxTransactionLimit,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay,TargetNullValue=''}" Height="25" VerticalContentAlignment="Center" TextChanged="TextBox_TextChanged" PreviewKeyDown="UpdateData">
                        <TextBox.Style>
                            <Style TargetType="{x:Type TextBox}">
                                <Style.Triggers>  
                                    <Trigger Property="Validation.HasError" Value="True">
                                        <Setter Property="ToolTip">
                                            <Setter.Value>
                                                <ToolTip DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget}">
                                                    <ItemsControl ItemsSource="{Binding Path=(Validation.Errors)}" DisplayMemberPath="ErrorContent" />
                                                </ToolTip>
                                            </Setter.Value>
                                        </Setter>
                                    </Trigger>
                                </Style.Triggers>
                            </Style>
                        </TextBox.Style>
                    </TextBox>
                    <TextBox Width="70" Text="{Binding LongPosition,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay,TargetNullValue=''}" Height="25" VerticalContentAlignment="Center" Margin="5,0,0,0" TextChanged="TextBox_TextChanged" PreviewKeyDown="UpdateData" >
                        <TextBox.Style>
                            <Style TargetType="{x:Type TextBox}">
                                <Style.Triggers>
                                    <!--<DataTrigger Binding="{Binding IsLimitExists}" Value="false">
                                        <Setter Property="Visibility" Value="Collapsed" />
                                    </DataTrigger>-->
                                    <Trigger Property="Validation.HasError" Value="True">
                                        <Setter Property="ToolTip">
                                            <Setter.Value>
                                                <ToolTip DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget}">
                                                    <ItemsControl ItemsSource="{Binding Path=(Validation.Errors)}" DisplayMemberPath="ErrorContent" />
                                                </ToolTip>
                                            </Setter.Value>
                                        </Setter>
                                    </Trigger>
                                </Style.Triggers>
                            </Style>
                        </TextBox.Style>
                    </TextBox>
                    <TextBox Width="70" Text="{Binding ShortPosition,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay,TargetNullValue=''}" Height="25" VerticalContentAlignment="Center" Margin="5,0,0,0" TextChanged="TextBox_TextChanged" PreviewKeyDown="UpdateData" >
                        <TextBox.Style>
                            <Style TargetType="{x:Type TextBox}">
                                <Style.Triggers>
                                    <!--<DataTrigger Binding="{Binding IsLimitExists}" Value="false">
                                        <Setter Property="Visibility" Value="Collapsed" />
                                    </DataTrigger>-->
                                    <Trigger Property="Validation.HasError" Value="True">
                                        <Setter Property="ToolTip">
                                            <Setter.Value>
                                                <ToolTip DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget}">
                                                    <ItemsControl ItemsSource="{Binding Path=(Validation.Errors)}" DisplayMemberPath="ErrorContent" />
                                                </ToolTip>
                                            </Setter.Value>
                                        </Setter>
                                    </Trigger>
                                </Style.Triggers>
                            </Style>
                        </TextBox.Style>
                    </TextBox>
                    <Button Content="Clear" Click="ClearGroupLimit" Width="50" Margin="5,0,0,0" Height="25" PreviewKeyDown="UpdateData"/>
                </StackPanel>
            </Border>
        </HierarchicalDataTemplate>

<treeview x:name="MainTreeView" horizontalalignment="Stretch" verticalalignment="Stretch" margin="0,0,0,0" itemssource="{Binding Limits}" grid.row="0" selecteditemchanged="MainTreeView_SelectedItemChanged">

c# code
private void MainTreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
var selectedTreeview = MainTreeView.ItemContainerGenerator.ContainerFromItem(((TreeView)sender).SelectedItem);
var selectedItem = ((TreeView)sender).SelectedItem;

if (selectedTreeview is TreeViewItem treeViewItem)
{
if (FindVisualChild<stackpanel>(treeViewItem as DependencyObject) is StackPanel stackPanel)
{
if (stackPanel.Children[0] is TextBox textbox)
{
textbox.Focus();
Keyboard.Focus(textbox);
e.Handled = true;
}
}
}
}