Hiding lines of code

Hi

What's the best way to hide several lines of code. I tried to hide the lines with

ICSharpCode.AvalonEdit.TextEditorTextArea.TextView.CollapseLines

But that results in an exception after clicking inside the text area:

System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=ICSharpCode.AvalonEdit
  StackTrace:
       at ICSharpCode.AvalonEdit.Rendering.TextView.GetOrConstructVisualLine(DocumentLine documentLine)
       at ICSharpCode.AvalonEdit.Editing.Caret.ValidateVisualColumn()
       at ICSharpCode.AvalonEdit.Editing.Caret.get_Position()
       at ICSharpCode.AvalonEdit.Editing.SelectionMouseHandler.textArea_MouseLeftButtonDown(Object sender, MouseButtonEventArgs e)
       at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
       at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)

...

Thanks for help...