Customized UserControl with ContentPresenter, nested objects are null

Question

I have a customized UserControl (CustomControl1) which I used as data container in MainPage.xaml.
As content of the MainPage- CustomControl1 I placed some buttons.

My error/question:
The buttons would be displayed, but I get an error when I try to set a button property.
Button object would be null…. But why?
In addition, no events would be raise from the nested objects, when I click on it in the development environment. Only if I select the nested object in XAML, I can use the [Object]->Properties->Events.

Silverlight Sources:
Vers.4
Vers.6 with TemplateBinding and Error

What must I change?
I think, it could be an error of my ContentPresenter binding in CustomControl1.

I found many pages with this kind of problem... But I couldn't fix my error...

My CustomControl1.xaml:

<UserControl.Resources>
    <Style x:Key="roundBorderStyle" TargetType="Border">
        <Setter Property="Background">
            <Setter.Value>
                <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                    <GradientStop Color="#FF5F0E8E" Offset="0"/>
                    <GradientStop Color="#FFA6CEF0" Offset="1"/>
                </LinearGradientBrush>
            </Setter.Value>
        </Setter>
        <Setter Property="BorderThickness" Value="3"/>
        <Setter Property="BorderBrush" Value="#FF0F2048"/>
        <Setter Property="VerticalAlignment" Value="Stretch"/>
    </Style>
</UserControl.Resources>
<Border Width="Auto" Style="{StaticResource roundBorderStyle}">
    <Grid x:Name="LayoutRoot">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid Grid.Row="0" Height="Auto">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <sdk:Label x:Name="FrameCaption" Content="Caption" VerticalAlignment="Center" HorizontalAlignment="Left" Grid.Row="0" />
            <Button x:Name="OpenButton" Content="Click" Grid.Column="2" VerticalAlignment="Center" HorizontalAlignment="Right" Margin="0,4,10,0" />
        </Grid>
        <ContentPresenter Grid.Row="1" x:Name="FrameContent" 
                            Content="{Binding NewContent, ElementName=userControl}"
                            Margin="4" Width="Auto" Height="Auto"/>
    </Grid>
</Border>

My CustomControl1.xaml.cs:

    public UIElement NewContent
    {
        get { return (UIElement)GetValue(NewContentProperty); }
        set { SetValue(NewContentProperty, value); }
    }

    public static readonly DependencyProperty NewContentProperty =
        DependencyProperty.Register("NewContent", typeof(UIElement), typeof(CustomControl1), null);

    public string Button2Content
    {
        set
        {
            // On run time myButton2 and Button2 are null
            // Because LayoutRoot_Loaded isn't finished.
            ((Button)LayoutRoot.FindName("Button2")).Content = value;
        }
        get
        {
            return myButton2.Content.ToString();
        }
    }

    private void LayoutRoot_Loaded(object sender, RoutedEventArgs e)
    {
        //Next step would throw an error, because Button1 is null ... Why?
        //Button1.Content = "NewContent"; 
        //Maybe a problem with ContentPresenter binding in CustomControl1?

        // This works ... But a little complicated...
        myButton1 = (Button)LayoutRoot.FindName("Button1");
        myButton2 = (Button)LayoutRoot.FindName("Button2");
        myButton3 = (Button)LayoutRoot.FindName("Button3");
        //myButton1.Content = "NewContent";
    }

My MainPage.xaml:

<my:CustomControl1 x:Name="LayoutRoot" Button2Content="New2Content" Loaded="LayoutRoot_Loaded">
    <StackPanel>
        <Button Content="Button 1" x:Name="Button1"/>
        <Button Content="Button 2" x:Name="Button2"/>
        <Button Content="Button 3" x:Name="Button3"/>
    </StackPanel>
</my:CustomControl1>