The Windows Presentation Framework object containing the pane's user interface. 
   Namespace:   Autodesk.Revit.UI  
  Assembly:   RevitAPIUI  (in RevitAPIUI.dll) Version: 18.0.0.0 (18.2.0.0) 
  Since:  2014 
Syntax
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Remarks
 Using a System.Windows.Controls.Page is recommended. This can be null, in which case it is assumed an IFrameworkElementCreator is provided to create the element on demand.