compose: Dispose topic-input's focus node when stream compose box disposed

Thanks Greg for catching this:
  zulip#1090 (comment)

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1137,6 +1137,7 @@ class _StreamComposeBoxState extends State<_StreamComposeBox> implements Compose
     _topicController.dispose();
     _contentController.dispose();
     _contentFocusNode.dispose();
+    _topicFocusNode.dispose();
     super.dispose();
   }