Added TreeView static ctor.

To work around SelectedItem not being found in Release build.

src/Perspex.Controls/TreeView.cs

@@ -27,6 +27,14 @@ namespace Perspex.Controls
 
         private object _selectedItem;
 
+        /// <summary>
+        /// Initializes static members of the <see cref="TreeView"/> class.
+        /// </summary>
+        static TreeView()
+        {
+            // HACK: Needed or SelectedItem property will not be found in Release build.
+        }
+
         /// <summary>
         /// Gets the <see cref="ITreeItemContainerGenerator"/> for the tree view.
         /// </summary>