StTest/MyTests/ErrorWindow.xaml

<Window x:Class="MyTests.ErrorWindow"
        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:MyTests"
        mc:Ignorable="d"
        Title="ErrorWindow" 
        MinHeight="120" 
        Height="120"
        Width="300" 
        WindowStartupLocation="CenterScreen" 
        WindowStyle="None" 
        ResizeMode="NoResize">
    <Grid Background="#FF2D2D2D">
        <TextBlock Name="ErrorLabel" 
               Text="Ошибка." 
               TextWrapping="Wrap"
               HorizontalAlignment="Center" 
               Margin="19,10,19,0" 
               VerticalAlignment="Top" 
               Width="262">
        </TextBlock>
        <Button Content="Закрыть" 
                VerticalAlignment="Bottom" 
                HorizontalAlignment="Center" 
                Height="30" 
                Width="130" 
                Margin="0,0,0,15" 
                Click="BackClick"/>
    </Grid>
</Window>