Value I'm trying to get is always null

Question

In my application I'm using a ribbon control, on this ribbon is a button. When I click the button it should get the selected value from the datagrid in my view.

Now when I click the button the value it gets is always null.

This is my code:

View1.xaml

<ScrollViewer Background="White" Grid.Row="1" Grid.Column="1" Margin="2" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Visible">
    <DataGrid SelectionMode="Single" ItemsSource="{Binding Collection}" SelectedItem="{Binding Test, Mode=TwoWay}" AutoGenerateColumns="False" Name="stackpanelLocked"  HorizontalAlignment="Left">
        <DataGrid.Columns>
            <DataGridTextColumn Header="RandomHeaderX" Binding="{Binding Path=X}"/>
            <DataGridTextColumn Header="RandomHeaderY" Binding="{Binding Path=Y}"/>
        </DataGrid.Columns>
    </DataGrid>
</ScrollViewer>

Viewmodel

//get selected item value
string lastSelectedValue;
private Model _test;        
public Model Test 
{
    get
    {
        return _test;
    }
    set
    {
        _test = value;
        if(Test != null)
            this.lastSelectedValue = string.Format("Variabele X: {0} Variabele Y: {1}", Test.X, Test.Y);
    }
}

private ICommand _ClickCommand;
public ICommand ClickCommand
{
    get
    {
        if (_ClickCommand == null)
            _ClickCommand = new DelegateCommand(new Action(Execute),
                        new Func<bool>(CanExecute));
        return _ClickCommand;
    }
}

public bool CanExecute()
{
    return true;
}

public void Execute()
{
    if(lastSelectedValue != null)
        Clipboard.SetText(lastSelectedValue);  
}

Model

public int X { get; set; }
public int Y { get; set; }

public Model(int x, int y)
{
    X = x;
    Y = y;
}

Extra info:

Every time I click in the datagrid it will actually fill the string lastSelectedValue. But when I click the button on the ribbon it will say that the lastSelectedValue is null.

Collection stuff:

public List<DatabaseClass.coord> AllCoords { get; set; }
public ObservableCollection<Model> Collection { get; set; }
public ModuleARibbonTabModel()
{
    Collection = new ObservableCollection<Model>();
    GenerateDatas();
}

private void GenerateDatas()
{
    using (var ctx = new DatabaseClass.TestDatabaseEntities1 ())
    {
        AllCoords = ctx.coord.ToList();
    }
    foreach(DatabaseClass.coord f in AllCoords)
    {
        Collection.Add(new Model(Convert.ToInt32(f.Een.ToString()), Convert.ToInt32(f.Twee.ToString())));
    }
}

Answer 1

I managed to work around the problem. I don't think this is a good fix but this is what I changed:
I changed the string lastSelectedValue to a static string lastSelectedValue This makes sure I can get the value.