Fix MultiSelectorHelper

Do not crash if new value is null. Instead do not apply the Binding

Author: timunie

src/MahApps.Metro/Controls/Helper/MultiSelectorHelper.cs

@@ -37,9 +37,12 @@ private static void OnSelectedItemsChanged(DependencyObject d, DependencyPropert
                 var oldBinding = GetSelectedItemBinding(d);
                 oldBinding?.UnBind();
 
-                var multiSelectorBinding = new MultiSelectorBinding((Selector)d, (IList)e.NewValue);
-                SetSelectedItemBinding(d, multiSelectorBinding);
-                multiSelectorBinding.Bind();
+                if (e.NewValue != null)
+                {
+                    var multiSelectorBinding = new MultiSelectorBinding((Selector)d, (IList)e.NewValue);
+                    SetSelectedItemBinding(d, multiSelectorBinding);
+                    multiSelectorBinding.Bind();
+                }
             }
         }