add null check to user list view

packages/stream_chat_flutter/lib/src/user_list_view.dart

@@ -160,9 +160,7 @@ class _UserListViewState extends State<UserListView>
 
   @override
   Widget build(BuildContext context) {
-    final child = ColoredBox(
-      color: UserListViewTheme.of(context).backgroundColor!,
-      child: UserListCore(
+    final userListCore = UserListCore(
       errorBuilder: widget.errorBuilder ??
           (BuildContext context, Object err) => _buildError(err),
       emptyBuilder: widget.emptyBuilder ?? (context) => _buildEmpty(),
@@ -189,9 +187,17 @@ class _UserListViewState extends State<UserListView>
       presence: widget.presence,
       groupAlphabetically: widget.groupAlphabetically,
       userListController: _userListController,
-      ),
     );
 
+    final backgroundColor = UserListViewTheme.of(context).backgroundColor;
+
+    final child = backgroundColor != null
+        ? ColoredBox(
+            color: backgroundColor,
+            child: userListCore,
+          )
+        : userListCore;
+
     if (!widget.pullToRefresh) {
       return child;
     } else {