Use MainActor Task instead of GCD.

Mac/MainWindow/AddFeed/AddFeedController.swift

@@ -66,11 +66,11 @@ import RSParser
 		}
 
 		account.createFeed(url: url.absoluteString, name: title, container: container, validateFeed: true) { result in
-			
-			DispatchQueue.main.async {
+
+			Task { @MainActor in
 				self.endShowingProgress()
 			}
-			
+
 			switch result {
 			case .success(let feed):
 				NotificationCenter.default.post(name: .UserDidAddFeed, object: self, userInfo: [UserInfoKey.feed: feed])
@@ -81,7 +81,7 @@ import RSParser
 				case AccountError.createErrorNotFound:
 					self.showNoFeedsErrorMessage()
 				default:
-					DispatchQueue.main.async {
+					Task { @MainActor in
 						NSApplication.shared.presentError(error)
 					}
 				}

Mac/MainWindow/MainWindowController.swift

@@ -106,11 +106,10 @@ enum TimelineSourceMode {
 		
 		NotificationCenter.default.addObserver(self, selector: #selector(articleThemeNamesDidChangeNotification(_:)), name: .ArticleThemeNamesDidChangeNotification, object: nil)
 		NotificationCenter.default.addObserver(self, selector: #selector(currentArticleThemeDidChangeNotification(_:)), name: .CurrentArticleThemeDidChangeNotification, object: nil)
-		
-		DispatchQueue.main.async {
+
+		Task { @MainActor in
 			self.updateWindowTitle()
 		}
-
 	}
 
 	// MARK: - API

Mac/MainWindow/OPML/ExportOPMLWindowController.swift

@@ -88,7 +88,7 @@ import Account
 		
 		panel.beginSheetModal(for: hostWindow!) { result in
 			if result == NSApplication.ModalResponse.OK, let url = panel.url {
-				DispatchQueue.main.async {
+				Task { @MainActor in
 					let filename = url.lastPathComponent
 					let opmlString = OPMLExporter.OPMLString(with: account, title: filename)
 					do {

Mac/MainWindow/Timeline/TimelineViewController.swift

@@ -320,7 +320,7 @@ final class TimelineViewController: NSViewController, UndoableCommandRunner, Unr
 			
 			if let selectedIndex = articles.firstIndex(where: { $0.articleID == articleID }) {
 				tableView.selectRow(selectedIndex)
-				DispatchQueue.main.async {
+				Task { @MainActor in
 					self.tableView.scrollTo(row: selectedIndex)
 				}
 				focus()