Fix AutoFillChildren property for UWP

Author: sourcechord

GridExtra.Shared/GridEx.cs

@@ -618,6 +618,11 @@ private static void OnAreaChanged(DependencyObject d, DependencyPropertyChangedE
 
             // 子要素全体のAutoFillを計算しなおす
             var grid = ctrl.Parent as Grid;
+            if (grid == null)
+            {
+                return;
+            }
+
             var isAutoFill = GetAutoFillChildren(grid);
             if (isAutoFill)
             {