Don't allow null logical or visual children.

And remove RedirectLogicalChildren which is no longer needed, or
desired.

src/Perspex.Controls/Control.cs

@@ -278,32 +278,14 @@ namespace Perspex.Controls
                 if (_logicalChildren == null)
                 {
                     var list = new PerspexList<ILogical>();
-                    LogicalChildren = list;
+                    list.ResetBehavior = ResetBehavior.Remove;
+                    list.Validate = ValidateLogicalChild;
+                    list.CollectionChanged += LogicalChildrenCollectionChanged;
+                    _logicalChildren = list;
                 }
 
                 return _logicalChildren;
             }
-
-            set
-            {
-                Contract.Requires<ArgumentNullException>(value != null);
-
-                if (_logicalChildren != value)
-                {
-                    if (_logicalChildren != null)
-                    {
-                        _logicalChildren.CollectionChanged -= LogicalChildrenCollectionChanged;
-                    }
-                }
-
-                if (value is PerspexList<ILogical>)
-                {
-                    ((PerspexList<ILogical>)value).ResetBehavior = ResetBehavior.Remove;
-                }
-
-                _logicalChildren = value;
-                _logicalChildren.CollectionChanged += LogicalChildrenCollectionChanged;
-            }
         }
 
         /// <summary>
@@ -520,15 +502,6 @@ namespace Perspex.Controls
             DataContextChanged?.Invoke(this, EventArgs.Empty);
         }
 
-        /// <summary>
-        /// Makes the control use a different control's logical children as its own.
-        /// </summary>
-        /// <param name="collection">The logical children to use.</param>
-        protected void RedirectLogicalChildren(IPerspexList<ILogical> collection)
-        {
-            LogicalChildren = collection;
-        }
-
         /// <summary>
         /// Called when the <see cref="DataContext"/> property begins and ends being notified.
         /// </summary>
@@ -566,6 +539,14 @@ namespace Perspex.Controls
             return null;
         }
 
+        private static void ValidateLogicalChild(ILogical c)
+        {
+            if (c == null)
+            {
+                throw new ArgumentException("Cannot add null to LogicalChildren.");
+            }
+        }
+
         private void LogicalChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
         {
             switch (e.Action)

src/Perspex.SceneGraph/Visual.cs

@@ -125,6 +125,7 @@ namespace Perspex
 
             var visualChildren = new PerspexList<IVisual>();
             visualChildren.ResetBehavior = ResetBehavior.Remove;
+            visualChildren.Validate = ValidateLogicalChild;
             visualChildren.CollectionChanged += VisualChildrenChanged;
             VisualChildren = visualChildren;
         }
@@ -457,6 +458,18 @@ namespace Perspex
             }
         }
 
+        /// <summary>
+        /// Ensures a visual child is not null.
+        /// </summary>
+        /// <param name="c">The visual child.</param>
+        private static void ValidateLogicalChild(IVisual c)
+        {
+            if (c == null)
+            {
+                throw new ArgumentNullException("Cannot add null to VisualChildren.");
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="RenderTransform"/>'s <see cref="Transform.Changed"/> event
         /// is fired.