Merge pull request #1035 from mpondo/FixMissingNullCheck

Add missing null checks to Bind

Author: vb2ae

src/Caliburn.Micro.Platform/Bind.cs

@@ -134,7 +134,7 @@ public static void SetModel(DependencyObject dependencyObject, object value)
 
         static void ModelChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
         {
-            if (View.InDesignMode || object.ReferenceEquals(e.NewValue, e.OldValue))
+            if (View.InDesignMode || e.NewValue == null || object.ReferenceEquals(e.NewValue, e.OldValue))
             {
                 return;
             }
@@ -165,7 +165,7 @@ static void ModelChanged(DependencyObject d, DependencyPropertyChangedEventArgs
 
         static void ModelWithoutContextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
         {
-            if (View.InDesignMode || object.ReferenceEquals(e.NewValue, e.OldValue))
+            if (View.InDesignMode || e.NewValue == null || object.ReferenceEquals(e.NewValue, e.OldValue))
             {
                 return;
             }