Selector: fixed possible null object exception.

Main/Source/ExtendedWPFToolkitSolution/Src/WPFToolkit.Extended/Core/Primitives/Selector.cs

@@ -323,6 +323,8 @@ namespace Microsoft.Windows.Controls.Primitives
         {
             _ignoreSetSelectedValue = true;
 
+            if (ItemsSource != null)
+            {
                 foreach (object item in ItemsSource)
                 {
                     var selectorItem = ItemContainerGenerator.ContainerFromItem(item) as SelectorItem;
@@ -332,6 +334,7 @@ namespace Microsoft.Windows.Controls.Primitives
                             selectorItem.IsSelected = false;
                     }
                 }
+            }
 
             _ignoreSetSelectedValue = false;
         }