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)

lib/base/settings_base_state.dart

@@ -4,13 +4,9 @@ import 'package:checker/db.dart';
 import 'package:checker/strings.dart';
 import 'package:flutter/material.dart';
 
-abstract class SettingsBaseState<T extends StatefulWidget>
-  extends BaseState<T> {
+abstract class SettingsBaseState<T extends StatefulWidget> extends BaseState<T> {
 
-  SettingsBaseState(SqliteHelper helper, String app) {
-    this.helper = helper;
-    this.app = app;
-  }
+  SettingsBaseState(SqliteHelper helper, String app) : super(helper, app);
 
   int selectedItem;