Try to fix SD2-1681 NullReferenceException in AvalonDock.DocumentContent.Close

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@5940 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Libraries/AvalonDock/AvalonDock/DocumentContent.cs

@@ -403,7 +403,7 @@ namespace AvalonDock
 
             //if documents are attached to an external source via DockingManager.DocumentsSource
             //let application host handle the document closing by itself
-            if (Manager.DocumentsSource != null)
+            if (Manager != null && Manager.DocumentsSource != null)
             {
                 Manager.HandleDocumentClose(this);
             }