Добавьте файлы проекта.

Windows/AlertWindow.xaml

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+<Window x:Class="GroundOrganizer.AlertWindow"
+        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:GroundOrganizer"
+        mc:Ignorable="d"
+        FontSize="14"
+        MaxHeight="900"  
+        MaxWidth="1920" 
+        SizeToContent="WidthAndHeight" 
+        ResizeMode="NoResize" 
+        WindowStartupLocation="CenterOwner" WindowStyle="ToolWindow" >
+    <Grid>
+        <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
+            <Label x:Name="alertLabel" Margin="10"/>
+            <Button x:Name="alertButtonOk" Content="Ok" 
+                    Margin="3" Width="75" 
+                    HorizontalAlignment="Center" 
+                    Click="Button_Click"/>
+        </StackPanel>
+        
+    </Grid>
+</Window>