Smoth screen changes. Step 1

lib/base/base_state.dart

@@ -25,8 +25,10 @@ abstract class BaseState<T extends StatefulWidget> extends State<T> {
   /// Введенное пользователем значение.
   String merchantID = '';
 
+  BaseState(this.helper, this.app);
+
   Widget getMainWidget() {
-    return app == null ? getBackground() : new Scaffold(appBar: getAppBar(),
+    return new Scaffold(appBar: getAppBar(),
         body: new Stack(children: <Widget>[
           getScreenContent(),
           new Center(child: loading ? new CircularProgressIndicator() : null)