I have recently started learning C# WPF UI and a bar (see below image) appears anytime I run my application. I may have done some reckless changes that may have caused the problem. Some advice?

Code:
<Window x:Class="WPFHello.MainWindow"
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:WPFHello"
mc:Ignorable="d"
Title="Hello" Height="450" Width="800">
<Grid>
<Label Content="Label" HorizontalAlignment="Left" VerticalAlignment="Top" RenderTransformOrigin="1.575,1.359" FontSize="20" Margin="10,10,0,0"/>
</Grid>