Don't re-create topic when clearing its messages.

Telegram/SourceFiles/history/history.cpp

@@ -189,7 +189,9 @@ void History::checkChatListMessageRemoved(not_null<HistoryItem*> item) {
 }
 
 void History::itemVanished(not_null<HistoryItem*> item) {
-	item->notificationThread()->removeNotification(item);
+	if (const auto thread = item->maybeNotificationThread()) {
+		thread->removeNotification(item);
+	}
 	if (lastKeyboardId == item->id) {
 		clearLastKeyboard();
 	}

Telegram/SourceFiles/history/history_item.cpp

@@ -1929,6 +1929,15 @@ not_null<Data::Thread*> HistoryItem::notificationThread() const {
 	return _history;
 }
 
+Data::Thread *HistoryItem::maybeNotificationThread() const {
+	if (const auto rootId = topicRootId()) {
+		if (const auto forum = _history->asForum()) {
+			return forum->topicFor(rootId);
+		}
+	}
+	return _history;
+}
+
 Data::ForumTopic *HistoryItem::topic() const {
 	if (const auto rootId = topicRootId()) {
 		if (const auto forum = _history->asForum()) {

Telegram/SourceFiles/history/history_item.h

@@ -214,6 +214,7 @@ public:
 	[[nodiscard]] TextWithEntities factcheckText() const;
 
 	[[nodiscard]] not_null<Data::Thread*> notificationThread() const;
+	[[nodiscard]] Data::Thread *maybeNotificationThread() const;
 	[[nodiscard]] not_null<History*> history() const {
 		return _history;
 	}

Telegram/SourceFiles/window/notifications_manager.cpp

@@ -290,7 +290,9 @@ System::SkipState System::skipNotification(
 	const auto item = notification.item;
 	const auto type = notification.type;
 	const auto messageType = (type == Data::ItemNotificationType::Message);
-	if (!item->notificationThread()->currentNotification()
+	const auto thread = item->maybeNotificationThread();
+	if (!thread
+		|| !thread->currentNotification()
 		|| (messageType && item->skipNotification())
 		|| (type == Data::ItemNotificationType::Reaction
 			&& skipReactionNotification(item))) {