All functions of the gui are working as expected, except one, after the user postpons the start of migration 3 times the postpone button and start button should disappear and start countdown should begin and display countdown of example 10 minutes, but the countdown timer is not displaying

Add-Type -AssemblyName PresentationFramework

function Show-GUI {
    [xml]$xaml = @"
<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Migration Tool" Height="450" Width="400" WindowStartupLocation="CenterScreen" Background="LightGray">
    <Grid Margin="10">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>

        <Image Source="C:MigrationGUI_LayoutsPic1.png" Width="100" Height="100" HorizontalAlignment="Center" Grid.Row="0"/>
        <TextBox Name="MessageTextBox" TextWrapping="Wrap" IsReadOnly="True" VerticalScrollBarVisibility="Auto" Margin="0,10,0,10" Grid.Row="1"/>
        
        <StackPanel Orientation="Vertical" HorizontalAlignment="Center" Grid.Row="2">
            <ProgressBar Name="ProgressBar" Height="20" Width="300" Minimum="0" Maximum="100" Value="0" Visibility="Collapsed" Margin="0,10,0,10"/>
            <TextBlock Name="CountdownTextBlock" Visibility="Collapsed" Foreground="Green" FontSize="16" HorizontalAlignment="Center"/>
        </StackPanel>

        <StackPanel Orientation="Vertical" HorizontalAlignment="Center" Grid.Row="3" Margin="0,0,0,10">
            <Button Name="StartButton" Content="Start" Width="100" Margin="5"/>
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
                <Button Name="PostponeButton" Content="Postpone" Width="70" Height="22" Margin="6"/>
                <ComboBox Name="TimeComboBox" Width="76" Margin="6">
                    <ComboBoxItem Content="15 Minutes" Tag="15"/>
                    <ComboBoxItem Content="30 Minutes" Tag="30"/>
                    <ComboBoxItem Content="1 Hour" Tag="60"/>
                </ComboBox>
            </StackPanel>
            <Button Name="RestartButton" Content="Restart" Width="100" Margin="5" Visibility="Collapsed"/>
        </StackPanel>

        <Image Source="C:MigrationGUI_LayoutsPic2.png" Width="100" Height="100" HorizontalAlignment="Center" Grid.Row="4"/>
    </Grid>
</Window>
"@

    $reader = (New-Object System.Xml.XmlNodeReader $xaml)
    $window = [Windows.Markup.XamlReader]::Load($reader)

    $window.FindName("MessageTextBox").Text = "Welcome to the Migration Tool"

    $startButton = $window.FindName("StartButton")
    $postponeButton = $window.FindName("PostponeButton")
    $timeComboBox = $window.FindName("TimeComboBox")
    $progressBar = $window.FindName("ProgressBar")
    $countdownTextBlock = $window.FindName("CountdownTextBlock")
    $restartButton = $window.FindName("RestartButton")
    $messageTextBox = $window.FindName("MessageTextBox")

    $postponeCount = 0

    $timer = New-Object System.Timers.Timer
    $timer.Interval = 1000
    $timer.AutoReset = $true
    $timer.Enabled = $false
    Register-ObjectEvent -InputObject $timer -EventName Elapsed -SourceIdentifier TimerElapsed -Action {
        [System.Windows.Application]::Current.Dispatcher.Invoke({
            $global:countdownTime--
            $countdownTextBlock.Text = "Starting in $global:countdownTime seconds..."
            if ($global:countdownTime -le 0) {
                $timer.Stop()
                $countdownTextBlock.Text = "Starting..."
                if (Check-Prerequisites) {
                    Start-Process
                }
            }
        })
    }

    $startButton.Add_Click({
        if (Check-Prerequisites) {
            Start-Process
        }
    })

    $postponeButton.Add_Click({
        $selectedItem = $timeComboBox.SelectedItem
        if ($selectedItem -ne $null) {
            $postponeMinutes = [int]$selectedItem.Tag
            Postpone $postponeMinutes
        } else {
            $messageTextBox.Text = "Please select a postpone time."
        }
    })

    $restartButton.Add_Click({
        Restart-Computer
    })

    function Postpone($minutes) {
        $postponeCount++
        if ($postponeCount -ge 3) {
            Hide-Buttons
            Start-Countdown
        } else {
            $window.Hide()
            #Start-Sleep -Seconds ($minutes * 60)
            Start-Sleep -Seconds ($minutes * 1)
            $window.ShowDialog() | Out-Null
        }
    }

    function Hide-Buttons {
        $startButton.Visibility = 'Collapsed'
        $postponeButton.Visibility = 'Collapsed'
        $timeComboBox.Visibility = 'Collapsed'
        $messageTextBox.Visibility = 'Collapsed'
    }

    function Start-Countdown {
        $countdownTextBlock.Visibility = 'Visible'
        $global:countdownTime = 600
        $timer.Start()
    }

    function Check-Prerequisites {
        $diskSpace = Get-PSDrive -PSProvider FileSystem | Where-Object {$_.Free -gt 5GB}
        if ($diskSpace -eq $null) {
            $messageTextBox.Text = "Error: Not enough disk space."
            return $false
        }

        $osVersion = [System.Environment]::OSVersion.Version
        $latestVersion = [Version]"10.0.19041.0"  # Example latest version
        if ($osVersion -lt $latestVersion) {
            $messageTextBox.Text = "Error: Windows version is not the latest."
            return $false
        }

        $internet = Test-Connection -ComputerName google.com -Count 1 -Quiet
        if (-not $internet) {
            $messageTextBox.Text = "Error: No internet connection."
            return $false
        }

        return $true
    }

    function Start-Process {
        $progressBar.Visibility = 'Visible'
        $progressBar.Value = 0
        $steps = 4

        for ($i = 1; $i -le $steps; $i++) {
            Start-Sleep -Seconds 2  # Simulate work being done
            $progressBar.Value = ($i / $steps) * 100
            $messageTextBox.Text = "Step $i of $steps completed."
        }

        $messageTextBox.Text = "Migration completed, please reboot your computer."
        $progressBar.Value = 100
        $restartButton.Visibility = 'Visible'
    }

    $window.ShowDialog() | Out-Null
}

Show-GUI

All functions of the gui are working as expected, except one, after the user postpons the start of migration 3 times the postpone button and start button should disappear and start countdown should begin and display countdown of example 10 minutes, but the countdown timer is not displaying