add default channel image to theme

example/lib/main.dart

@@ -28,6 +28,13 @@ class MyApp extends StatelessWidget {
       themeMode: ThemeMode.system,
       home: Container(
         child: StreamChat(
+          streamChatThemeData: StreamChatThemeData(
+            defaultChannelImage: (context, channel) {
+              return Center(
+                child: Text('AAA'),
+              );
+            },
+          ),
           client: client,
           child: ChannelListPage(),
         ),

lib/src/channel_header.dart

@@ -128,7 +128,7 @@ class ChannelHeader extends StatelessWidget implements PreferredSizeWidget {
                     .channelHeaderTheme
                     .lastMessageAt,
               )
-            : Container();
+            : SizedBox();
       },
     );
   }

lib/src/channel_image.dart

@@ -111,7 +111,8 @@ class ChannelImage extends StatelessWidget {
                           },
                           fit: BoxFit.cover,
                         )
-                      : SizedBox(),
+                      : StreamChatTheme.of(context)
+                          .defaultChannelImage(context, channel),
                   Material(
                     color: Colors.transparent,
                     child: InkWell(

lib/src/stream_chat.dart

@@ -114,6 +114,7 @@ class StreamChatState extends State<StreamChat>
     final defaultTheme = StreamChatThemeData.getDefaultTheme(Theme.of(context));
     final theme = defaultTheme.copyWith(
       primaryColor: themeData?.primaryColor,
+      defaultChannelImage: themeData?.defaultChannelImage,
       channelTheme: defaultTheme.channelTheme.copyWith(
         channelHeaderTheme:
             defaultTheme.channelTheme.channelHeaderTheme.copyWith(

lib/src/stream_chat_theme.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:stream_chat/stream_chat.dart';
 import 'package:stream_chat_flutter/src/channel_header.dart';
 import 'package:stream_chat_flutter/src/channel_preview.dart';
 import 'package:stream_chat_flutter/src/message_input.dart';
@@ -50,6 +51,9 @@ class StreamChatThemeData {
   /// Theme of other users messages
   final MessageTheme otherMessageTheme;
 
+  /// The widget that will be built when the channel image is unavailable
+  final Widget Function(BuildContext, Channel) defaultChannelImage;
+
   /// Create a theme from scratch
   StreamChatThemeData({
     this.primaryColor,
@@ -59,6 +63,7 @@ class StreamChatThemeData {
     this.channelTheme,
     this.otherMessageTheme,
     this.ownMessageTheme,
+    this.defaultChannelImage,
   });
 
   /// Create a theme from a Material [Theme]
@@ -97,11 +102,13 @@ class StreamChatThemeData {
     ChannelTheme channelTheme,
     MessageTheme ownMessageTheme,
     MessageTheme otherMessageTheme,
+    Widget Function(BuildContext, Channel) defaultChannelImage,
   }) =>
       StreamChatThemeData(
         primaryColor: primaryColor ?? this.primaryColor,
         secondaryColor: secondaryColor ?? this.secondaryColor,
         accentColor: accentColor ?? this.accentColor,
+        defaultChannelImage: defaultChannelImage ?? this.defaultChannelImage,
         channelPreviewTheme: channelPreviewTheme?.copyWith(
               title:
                   channelPreviewTheme.title ?? this.channelPreviewTheme.title,
@@ -170,6 +177,7 @@ class StreamChatThemeData {
     return StreamChatThemeData(
       accentColor: accentColor,
       primaryColor: isDark ? Colors.black : Colors.white,
+      defaultChannelImage: (context, channel) => SizedBox(),
       channelPreviewTheme: ChannelPreviewTheme(
         avatarTheme: AvatarTheme(
           borderRadius: BorderRadius.circular(20),