Fix scroll stutter when URL previews fail to load

commet/lib/client/components/url_preview/url_preview_component.dart

@@ -14,16 +14,23 @@ abstract class UrlPreviewComponent<T extends Client> implements Component<T> {
   Future<UrlPreviewData?> getPreviewForUrl(Room room, Uri url);
 
   UrlPreviewData? getCachedPreview(Timeline timeline, TimelineEvent event);
+
+  // This is a dummy value that we can store in the cache when we fail to get the url preview.
+  // Instead of storing null, which is a bit confusing as if the cache returns null, does that mean we have
+  // nothing cached, or the result was invalid? maybe this is dumb but it was the simplest way
+  // to prevent weird ui glitches when failed fetches kept getting retried.
+  static UrlPreviewData invalidPreviewData =
+      UrlPreviewData(Uri.new(), title: "Unable to get url preview ");
 }
 
 class UrlPreviewData {
-  Uri uri;
-  String? siteName;
-  String? title;
-  String? description;
-  ImageProvider? image;
+  final Uri uri;
+  final String? siteName;
+  final String? title;
+  final String? description;
+  final ImageProvider? image;
 
-  UrlPreviewData(
+  const UrlPreviewData(
     this.uri, {
     this.siteName,
     this.title,

commet/lib/client/matrix/components/url_preview/matrix_url_preview_component.dart

@@ -56,6 +56,8 @@ class MatrixUrlPreviewComponent implements UrlPreviewComponent<MatrixClient> {
 
     if (data != null) {
       cache[uri.toString()] = data;
+    } else {
+      cache[uri.toString()] = UrlPreviewComponent.invalidPreviewData;
     }
 
     return data;

commet/lib/ui/molecules/timeline_events/events/timeline_event_view_url_previews.dart

@@ -33,6 +33,9 @@ class _TimelineEventViewUrlPreviewsState
   @override
   Widget build(BuildContext context) {
     BenchmarkValues.numTimelineUrlPreviewBuilt += 1;
+
+    if (data == UrlPreviewComponent.invalidPreviewData) return Container();
+
     return Padding(
       padding: const EdgeInsets.fromLTRB(0, 2, 40, 2),
       child: (loading || data != null)