add null check to message search list view

packages/stream_chat_flutter/lib/src/message_search_list_view.dart

@@ -144,9 +144,8 @@ class _MessageSearchListViewState extends State<MessageSearchListView> {
       widget.messageSearchListController ?? _defaultController;
 
   @override
-  Widget build(BuildContext context) => ColoredBox(
-        color: MessageSearchListViewTheme.of(context).backgroundColor!,
-        child: MessageSearchListCore(
+  Widget build(BuildContext context) {
+    final messageSearchListCore = MessageSearchListCore(
       filters: widget.filters,
       sortOptions: widget.sortOptions,
       messageQuery: widget.messageQuery,
@@ -197,9 +196,18 @@ class _MessageSearchListViewState extends State<MessageSearchListView> {
                 ),
               ),
       childBuilder: widget.childBuilder ?? _buildListView,
-        ),
     );
 
+    final backgroundColor =
+        MessageSearchListViewTheme.of(context).backgroundColor;
+    return backgroundColor != null
+        ? ColoredBox(
+            color: backgroundColor,
+            child: messageSearchListCore,
+          )
+        : messageSearchListCore;
+  }
+
   Widget _separatorBuilder(BuildContext context, int index) => Container(
         height: 1,
         color: StreamChatTheme.of(context).colorTheme.borders,