Simple Button.Text not changing properly

Question

Hi all,
The WPF learning curve is steep.
I have pulled my hair over this simple Button.Text change and I can't get it to work.
In WinForms this works:
Public Class Form1
Private Sub BTN_1_Click(sender As Object, e As EventArgs) Handles BTN_1.Click
BTN_1.Text = "CLICKED BUTTON 1"
BTN_2.Text = "CHANGED BY BUTTON 1"
End Sub
Private Sub BTN_2_Click(sender As Object, e As EventArgs) Handles BTN_2.Click
BTN_2.Text = "CLICKED BUTTON 2"
BTN_1.Text = "CHANGED BY BUTTON 2"
End Sub
End Class
I want to do the same thing in WPF
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Grid Grid.Column="0" Grid.Row="0">
<Button Name="BTN_1" Click="CLICK_BTN_1">
<StackPanel>
<TextBlock Name="CLICK_BTN_1_LABEL_1" Text="Button 1 text" VerticalAlignment="Top" Foreground="#BF000000" >
</TextBlock>
<TextBlock Name="CLICK_BTN_1_LABEL_2" VerticalAlignment="Top" Foreground="#BF000000">
<Run FontSize="25">Button 1</Run>
<Run FontSize="12">Label 2</Run>
</TextBlock>
</StackPanel>
</Button>
</Grid>
<Grid Grid.Column="1" Grid.Row="0">
<Button Name="BTN_2" Click="CLICK_BTN_1">
<StackPanel>
<TextBlock Name="CLICK_BTN_2_LABEL_1" Text="Button 2 text" VerticalAlignment="Top" Foreground="#BF000000">
</TextBlock>
<TextBlock Name="CLICK_BTN_2_LABEL_2" VerticalAlignment="Top" Foreground="#BF000000">
<Run FontSize="25">Button 2</Run>
<Run FontSize="12">Label 2</Run>
</TextBlock>
</StackPanel>
</Button>
</Grid>
</Grid>
</Window>
Class MainWindow
Private Sub CLICK_BTN_1(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles BTN_1.Click
' CHANGE LABEL 1 ON BUTTON 1
CLICK_BTN_1_LABEL_1.Text = "CLICKED BUTTON 1"
' CHANGE LABEL 1 ON BUTTON 2
CLICK_BTN_2_LABEL_1.Text = "CHANGED BY BUTTON 1"
End Sub
Private Sub CLICK_BTN_2(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles BTN_2.Click
' CHANGE LABEL 1 ON BUTTON 1
CLICK_BTN_1_LABEL_1.Text = "CHANGED BY BUTTON 2"
' CHANGE LABEL 1 ON BUTTON 2
CLICK_BTN_2_LABEL_1.Text = "CLICKED BUTTON 2"
End Sub
End Class
This used to be so simple with WinForms, what am I doing wrong here?
New to WPF

Answer 1

Yeah, I saw that as well but now I am stuck again:
Same project but I have put the buttons in a usercontrol
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:Component_Changes"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<Grid Grid.Column="0" Grid.Row="0">
<Viewbox Stretch="Fill">
<ContentControl Name="UC_BTN1">
<ContentControl.Content>
<local:btn1 Margin="20"/>
</ContentControl.Content>
</ContentControl>
</Viewbox>
</Grid>
<Grid Grid.Column="1" Grid.Row="0">
<Viewbox Stretch="Fill">
<ContentControl Name="UC_BTN2">
<ContentControl.Content>
<local:btn2 Margin="20"/>
</ContentControl.Content>
</ContentControl>
</Viewbox>
</Grid>
</Grid>
</Window>
btn1.xaml
<UserControl x:Class="btn1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
<Button Name="BTN_1" Click="CLICK_BTN_1">
<StackPanel>
<TextBlock Name="CLICK_BTN_1_LABEL_1" Text="Button 1 text" VerticalAlignment="Top" Foreground="#BF000000" >
</TextBlock>
<TextBlock Name="CLICK_BTN_1_LABEL_2" VerticalAlignment="Top" Foreground="#BF000000">
<Run FontSize="25">Button 1</Run>
<Run FontSize="12">Label 2</Run>
</TextBlock>
</StackPanel>
</Button>
</Grid>
</UserControl>
btn1.xaml.vb
Public Class btn1
Dim CL_BTN2 As btn2
Private Sub CLICK_BTN_1(sender As Object, e As RoutedEventArgs) Handles BTN_1.Click
' CHANGE LABEL 1 ON BUTTON 1
CLICK_BTN_1_LABEL_1.Text = "CLICKED BUTTON 1"
' CHANGE LABEL 1 ON BUTTON 2
CL_BTN2.CLICK_BTN_2_LABEL_1.Text = "CHANGED BY BUTTON 1"
End Sub
End Class
btn2.xaml
<UserControl x:Class="btn2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300">
<Grid>
<Button Name="BTN_2" Click="CLICK_BTN_2">
<StackPanel>
<TextBlock Name="CLICK_BTN_2_LABEL_1" Text="Button 2 text" VerticalAlignment="Top" Foreground="#BF000000">
</TextBlock>
<TextBlock Name="CLICK_BTN_2_LABEL_2" VerticalAlignment="Top" Foreground="#BF000000">
<Run FontSize="25">Button 2</Run>
<Run FontSize="12">Label 2</Run>
</TextBlock>
</StackPanel>
</Button>
</Grid>
</UserControl>
btn2.xaml.vb
Public Class btn2
Dim CL_BTN1 As btn1
Private Sub CLICK_BTN_2(sender As Object, e As RoutedEventArgs) Handles BTN_2.Click
' CHANGE LABEL 1 ON BUTTON 1
CL_BTN1.CLICK_BTN_1_LABEL_1.Text = "CHANGED BY BUTTON 2"
' CHANGE LABEL 1 ON BUTTON 2
CLICK_BTN_2_LABEL_1.Text = "CLICKED BUTTON 2"
End Sub
End Class
No warnings in the code but the error is: 'Object reference not set to an instance of an object' and I thought I did, obviously not in the right way.
New to WPF