🐛 Fix arguments judging condition with preview thumb size.

Resolves #60 .

Author: AlexV525

lib/src/constants/constants.dart

@@ -27,6 +27,8 @@ class Constants {
   static AssetsPickerTextDelegate textDelegate =
       DefaultAssetsPickerTextDelegate();
   static SortPathDelegate sortPathDelegate = SortPathDelegate.common;
+
+  static const List<int> defaultPreviewThumbSize = <int>[200, 200];
 }
 
 /// Log only in debug mode.

lib/src/provider/asset_entity_image_provider.dart

@@ -8,21 +8,22 @@ import 'dart:ui' as ui;
 import 'package:flutter/material.dart';
 import 'package:flutter/foundation.dart';
 import 'package:photo_manager/photo_manager.dart';
+import 'package:wechat_assets_picker/src/constants/constants.dart';
 
 @immutable
 class AssetEntityImageProvider extends ImageProvider<AssetEntityImageProvider> {
   AssetEntityImageProvider(
     this.entity, {
     this.scale = 1.0,
-    this.thumbSize = const <int>[200, 200],
+    this.thumbSize = Constants.defaultPreviewThumbSize,
     this.isOriginal = true,
   }) : assert(
-          thumbSize?.length == 2,
-          'thumbSize must contain and only contain two integers',
+          isOriginal || thumbSize?.length == 2,
+          'thumbSize must contain and only contain two integers when it\'s not original',
         ) {
-    if (thumbSize?.length != 2) {
+    if (!isOriginal && thumbSize?.length != 2) {
       throw ArgumentError(
-        'thumbSize must contain and only contain two integers',
+        'thumbSize must contain and only contain two integers when it\'s not original',
       );
     }
   }

lib/src/widget/builder/audio_page_builder.dart

@@ -6,9 +6,9 @@ import 'dart:async';
 
 import 'package:flutter/material.dart';
 import 'package:video_player/video_player.dart';
+import 'package:wechat_assets_picker/wechat_assets_picker.dart';
 
 import 'package:wechat_assets_picker/src/constants/constants.dart';
-import 'package:wechat_assets_picker/wechat_assets_picker.dart';
 
 class AudioPageBuilder extends StatefulWidget {
   const AudioPageBuilder({