Make it non-nullable

Author: mzdm

README.md

@@ -137,7 +137,7 @@ You should then run `flutter packages get` in your terminal so as to get the pac
 | Dart attribute        | Datatype       | Description                                                           |                                          Default Value                                          |
 | :-------------------- | :------------- | :-------------------------------------------------------------------- | :---------------------------------------------------------------------------------------------: |
 | pageColor             | Color?          | Set color of the page.                                                |                                              Null                                               |
-| pageBackground        | Widget?         | Set a widget as a background of the whole page (pageColor has priority)            |                                              Null                                               |
+| pageBackground        | Widget         | Set a widget as a background of the whole page (pageColor has priority)            |                                              Null                                               |
 | mainImage             | Image? / Widget? | Set the main image of the page.                                       |                                              Null                                               |
 | title                 | Text? / Widget?  | Set the title text of the page. If null, then the widget is omitted.                                       |                                              Null                                               |
 | body                  | Text? / Widget?  | Set the body text of the page. If null, then the widget is omitted.                                        |                                              Null                                               |

lib/src/models/page_view_model.dart

@@ -4,7 +4,7 @@ import 'package:flutter/material.dart';
 class PageViewModel {
   const PageViewModel({
     this.pageColor,
-    this.pageBackground,
+    this.pageBackground = const SizedBox(),
     this.iconImageAssetPath,
     this.bubbleBackgroundColor = const Color(0x88FFFFFF),
     this.iconColor,
@@ -27,7 +27,7 @@ class PageViewModel {
   /// or adding a background image.
   ///
   /// [pageColor] has priority over [pageBackground].
-  final Widget? pageBackground;
+  final Widget pageBackground;
 
   /// Icon image path.
   final String? iconImageAssetPath;

lib/src/ui/page.dart

@@ -22,7 +22,7 @@ class Page extends StatelessWidget {
   Widget build(BuildContext context) {
     return Stack(
       children: <Widget>[
-        pageViewModel.pageBackground ?? const SizedBox(),
+        pageViewModel.pageBackground,
         Container(
           padding: const EdgeInsets.all(8.0),
           width: double.infinity,