Index: Core/Common/src/Core.Common.Gui/Forms/ViewHost/DocumentViewController.cs
===================================================================
diff -u -r283d4d7b66e4464498c934b787e84cc4e27857de -r81bfdfc84c3fc00dbfec60ac1863a69bfd03a33c
--- Core/Common/src/Core.Common.Gui/Forms/ViewHost/DocumentViewController.cs	(.../DocumentViewController.cs)	(revision 283d4d7b66e4464498c934b787e84cc4e27857de)
+++ Core/Common/src/Core.Common.Gui/Forms/ViewHost/DocumentViewController.cs	(.../DocumentViewController.cs)	(revision 81bfdfc84c3fc00dbfec60ac1863a69bfd03a33c)
@@ -126,7 +126,12 @@
 
         private void ViewHostOnViewClosed(object sender, ViewChangeEventArgs viewChangeEventArgs)
         {
-            openedViewLookup.Remove(openedViewLookup.First(l => ReferenceEquals(l.Value, viewChangeEventArgs.View)).Key);
+            KeyValuePair<object, IView> openViewItem = openedViewLookup.FirstOrDefault(l => ReferenceEquals(l.Value, viewChangeEventArgs.View));
+
+            if (!openViewItem.Equals(default(KeyValuePair<object, IView>)))
+            {
+                openedViewLookup.Remove(openViewItem.Key);
+            }
         }
 
         private bool ShouldRemoveViewForData(IView view, object data)