remove assert for nullable constructor params

packages/stream_chat_flutter_core/lib/src/channels_bloc.dart

@@ -25,8 +25,7 @@ class ChannelsBloc extends StatefulWidget {
     this.lockChannelsOrder = false,
     this.channelsComparator,
     this.shouldAddChannel,
-  })  : assert(child != null, 'Parameter child should not be null.'),
+  }) : super(key: key);
-        super(key: key);
 
   /// The widget child
   final Widget child;

packages/stream_chat_flutter_core/lib/src/message_list_core.dart

@@ -70,17 +70,7 @@ class MessageListCore extends StatefulWidget {
     this.parentMessage,
     this.messageListController,
     this.messageFilter,
-  })  : assert(loadingBuilder != null, 'loadingBuilder should not be null'),
+  }) : super(key: key);
-        assert(emptyBuilder != null, 'emptyBuilder should not be null'),
-        assert(
-          messageListBuilder != null,
-          'messageListBuilder should not be null',
-        ),
-        assert(
-          errorWidgetBuilder != null,
-          'errorWidgetBuilder should not be null',
-        ),
-        super(key: key);
 
   /// A [MessageListController] allows pagination.
   /// Use [ChannelListController.paginateData] pagination.

packages/stream_chat_flutter_core/lib/src/message_search_bloc.dart

@@ -15,8 +15,7 @@ class MessageSearchBloc extends StatefulWidget {
   const MessageSearchBloc({
     Key? key,
     required this.child,
-  })   : assert(child != null, 'Parameter child should not be null.'),
+  }) : super(key: key);
-        super(key: key);
 
   /// The widget child
   final Widget child;