add null check to user list view

packages/stream_chat_flutter/lib/src/user_list_view.dart

@@ -160,38 +160,44 @@ class _UserListViewState extends State<UserListView>
 
   @override
   Widget build(BuildContext context) {
-    final child = ColoredBox(
-      color: UserListViewTheme.of(context).backgroundColor!,
-      child: UserListCore(
-        errorBuilder: widget.errorBuilder ??
-            (BuildContext context, Object err) => _buildError(err),
-        emptyBuilder: widget.emptyBuilder ?? (context) => _buildEmpty(),
-        loadingBuilder: widget.loadingBuilder ??
-            (context) => LayoutBuilder(
-                  builder: (context, viewportConstraints) =>
-                      SingleChildScrollView(
-                    physics: const AlwaysScrollableScrollPhysics(),
-                    child: ConstrainedBox(
-                      constraints: BoxConstraints(
-                        minHeight: viewportConstraints.maxHeight,
-                      ),
-                      child: const Center(
-                        child: CircularProgressIndicator(),
-                      ),
+    final userListCore = UserListCore(
+      errorBuilder: widget.errorBuilder ??
+          (BuildContext context, Object err) => _buildError(err),
+      emptyBuilder: widget.emptyBuilder ?? (context) => _buildEmpty(),
+      loadingBuilder: widget.loadingBuilder ??
+          (context) => LayoutBuilder(
+                builder: (context, viewportConstraints) =>
+                    SingleChildScrollView(
+                  physics: const AlwaysScrollableScrollPhysics(),
+                  child: ConstrainedBox(
+                    constraints: BoxConstraints(
+                      minHeight: viewportConstraints.maxHeight,
+                    ),
+                    child: const Center(
+                      child: CircularProgressIndicator(),
                     ),
                   ),
                 ),
-        listBuilder:
-            widget.listBuilder ?? (context, list) => _buildListView(list),
-        pagination: widget.pagination,
-        sort: widget.sort,
-        filter: widget.filter,
-        presence: widget.presence,
-        groupAlphabetically: widget.groupAlphabetically,
-        userListController: _userListController,
-      ),
+              ),
+      listBuilder:
+          widget.listBuilder ?? (context, list) => _buildListView(list),
+      pagination: widget.pagination,
+      sort: widget.sort,
+      filter: widget.filter,
+      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 {