SourceTree won't start

ERROR [2017-10-09 07:53:08,277] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-10-09 07:53:08,418] [1] [SourceTree.App] [OnStartup] - Failed to start
System.NullReferenceException: Odwołanie do obiektu nie zostało ustawione na wystąpienie obiektu.
   w SourceTree.Notifications.NotificationsManager.SetOwner(NotificationDialogWindow notificationWindow)
   w SourceTree.Notifications.NotificationsManager.ShowNotificationDialog[T](NotificationDialogWindow notificationWindow, Tuple`2 customAction, VistaTaskDialogIcon icon)
   w SourceTree.Notifications.NotificationsManager.ShowNotificationDialog[T](String title, String message, Tuple`2 customAction, String cancelLabel, String suppressionSetting, Action`1 suppressionChangedAction, Object contentControl, String contentCommandLabel, Action contentAction)
   w SourceTree.Notifications.NotificationsManager.ShowNotificationDialogWithYesConfirmation(String title, String message, String details)
   w SourceTree.Configuration.WpfSpellCheckerPreFlightCheck.Run()
   w SourceTree.AppRoot.RunPreFlightChecks()
   w SourceTree.AppRoot.OnStartup(StartupEventArgs e)
   w SourceTree.App.OnStartup(StartupEventArgs e)