fix last active on new message

lib/src/channel_header.dart

@@ -120,15 +120,17 @@ class ChannelHeader extends StatelessWidget implements PreferredSizeWidget {
       stream: channel.lastMessageAtStream,
       initialData: channel.lastMessageAt,
       builder: (context, snapshot) {
-        return (snapshot.data != null)
+        if (snapshot.data == null) {
-            ? Text(
+          return SizedBox();
-                'Active ${Jiffy(snapshot.data.toLocal()).fromNow()}',
+        }
+        final jiffyDate = Jiffy(snapshot.data?.toLocal());
+        return Text(
+          'Active ${jiffyDate.isBefore(Jiffy()) ? jiffyDate.fromNow() : 'now'}',
           style: StreamChatTheme.of(context)
               .channelTheme
               .channelHeaderTheme
               .lastMessageAt,
-              )
+        );
-            : SizedBox();
       },
     );
   }