<dxdo:DockLayoutManager Margin="0,156,0,20" x:Name="dockManager_plan"
 FlowDirection="RightToLeft" AutoHideExpandMode="MouseDown" 
MouseLeftButtonDown="dockManager_plan_MouseLeftButtonDown">
            <dxdo:LayoutGroup Orientation="Horizontal" CaptionHorizontalAlignment="Left">
                <dxdo:DocumentGroup CaptionLocation="Bottom"  Name="documentContainer_plan" 
                                    ClosePageButtonShowMode="InAllTabPageHeaders">
                    <dxdo:DocumentPanel x:Name="paneDocument_plan_1" Caption="Document 1">
                        <Grid>
                            <Button Name="button1" Click="button1_Click"  Content="button" Width="100" Height="19" />
                        </Grid>
                    </dxdo:DocumentPanel>
                </dxdo:DocumentGroup>
            </dxdo:LayoutGroup>

   private void button1_Click(object sender, RoutedEventArgs e)
        {
          button1.Content="Test";
        }

 if (button1 == null)

{dxdo:RelativeUri UriString

label1.Content = docPanel.Content;