Setting DocumentSource throws NullReferenceException when PdfViewer is hosted in ElementHost. When setting the DocumentSource, the Application.Dispatcher is used, but in Winforms app, the Application is not instantiated. 

Steps to reproduce:
1. Host PdfViewer in ElementHost in a WinForms application
2. Set the DocumentSource property
3. Run
Expected: The document is opened
Actual: NullReferenceException is thrown

Workaround: the document could be set to the Document property of PdfViewer. Or instead hosting the PdfViewer for WPF in ElementHost, use WinForms version of the control.