Fixes for rare AvalonDock bugs:

- Exception when closing tab by middle-clicking
 - Exception when hovering over auto-hide pad

src/Libraries/AvalonDock/AvalonDock/DockingManager.cs

@@ -2636,6 +2636,15 @@ namespace AvalonDock
                 //_flyoutWindow.KeepWindowOpen();
                 return;
             }
+            
+            // David: This sometimes seems to be null when hovering over auto-hide pad.
+            // It only seems to occur in specific circumstances - whether the pad is the
+            // "selected" one in the set seems important.  Having no open document might have effect?
+            // I have only managed to reliablely reproduce it in stand-alone application.
+			if (content.ContainerPane == null)
+			{
+				return;
+			}
 
             //hide previous window
             HideFlyoutWindow();

src/Libraries/AvalonDock/AvalonDock/DocumentTabPanel.cs

@@ -65,6 +65,10 @@ namespace AvalonDock
 
             foreach (ManagedContent child in Children)
             {
+				// David: Sometimes happened when closing document tab by middle clicking.  Really difficult to reproduce.
+				if (child == null)
+					continue;
+				
                 Panel.SetZIndex(child, Selector.GetIsSelected(child) ? 1 : -i);
                 i++;
                 child.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
@@ -81,6 +85,10 @@ namespace AvalonDock
             bool skipAllOthers = false;
             foreach (ManagedContent doc in Children)
             {
+				// David: Sometimes happened when closing document tab by middle clicking.  Really difficult to reproduce.
+				if (doc == null)
+					continue;
+				
                 if (skipAllOthers || offset + doc.DesiredSize.Width > finalSize.Width)
                 {
                     SetIsHeaderVisible(doc, false);