Merge branch 'kgdev' into 'master'

fix bugs.

See merge request upm-packages/ui-widgets/com.unity.uiwidgets!108

Runtime/material/app.cs

             string title = "",
             Color color = null,
             ThemeData theme = null,
-            bool showPerformanceOverlay = false,
-            Window window = null) : base(key: key) {
-            D.assert(window != null);
-            this.window = window;
+            bool showPerformanceOverlay = false
+        ) : base(key: key) {
             this.navigatorKey = navigatorKey;
             this.home = home;
             this.routes = routes ?? new Dictionary<string, WidgetBuilder>();
         public readonly Color color;
 
         public readonly bool showPerformanceOverlay;
-
-        public readonly Window window;
-
 
         public override State createState() {
             return new _MaterialAppState();

Runtime/widgets/pages.cs

         }
 
         public override bool opaque {
-            get { return false; }
+            get { return true; }
         }
 
         public override bool barrierDismissible {