DispatchHelper open new window - wpf

Question

I am using the DispatchHelper from MVVM Light to open a new window while a calculation is being performed. The window contains a simple label and button. When I use the Show() method the window will appear when the calculation starts and will close when the calculation ends but the label and button are not in the window.
I have tried using the ShowDialog() method before the calculation starts and does show the label and button on the window. However, the ShowDialog doesn't return until the window gets closed. Therefore, the window stay open and the calculation does not get performed.
Would anyone have some advise on how to work through this issue?
public RelayCommand ShowPopupControl
{
get
{
return _showPopupControl
?? (_showPopupControl = new RelayCommand(
() => {
DispatcherHelper.CheckBeginInvokeOnUI(() =>
{
var ww = new WorkingWindow();
ww.ShowDialog();
var loop = new LoopData();
loop.DoSomeLoops();
ww.Close();
});
}));
}
}
public class LoopData
{
public void DoSomeLoops()
{
for (int i = 0; i < 10000; i++)
{
System.Diagnostics.Debug.WriteLine(i);
}
}
}
<controls:MetroWindow
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:Custom="http://metro.mahapps.com/winfx/xaml/controls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
x:Class="MetroSpinnerPractice.WorkingWindow"
xmlns:controls="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
Title="WorkingWindow" Height="200" Width="300">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<StackPanel HorizontalAlignment="Center" Height="75" Margin="5" Grid.Row="0" VerticalAlignment="Center" Width="275" Orientation="Horizontal">
<Label Content="Working...."
VerticalAlignment="Center"
Margin="60,20,0,20"
FontSize="21.333"
HorizontalAlignment="Right"
Width="120"
Height="40"/>
</StackPanel>
<Button x:Name="btnCancelWork" Grid.Row="1"
Content="Cancel"
Width="100"
Height="20"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Margin="0"
ToolTip="cancel current routine"/>
</Grid>