SourceTree failes to start (NullReferenceException in stack trace)

ERROR [2017-09-14 12:42:05,554] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-09-14 12:42:05,761] [1] [SourceTree.App] [OnStartup] - Failed to start
System.NullReferenceException: Object reference not set to an instance of an object.
   at SourceTree.Notifications.NotificationsManager.SetOwner(NotificationDialogWindow notificationWindow)
   at SourceTree.Notifications.NotificationsManager.ShowNotificationDialog[T](NotificationDialogWindow notificationWindow, Tuple`2 customAction, VistaTaskDialogIcon icon)
   at 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)
   at SourceTree.Notifications.NotificationsManager.ShowNotificationDialogWithYesConfirmation(String title, String message, String details)
   at SourceTree.Configuration.WpfSpellCheckerPreFlightCheck.Run()
   at SourceTree.AppRoot.RunPreFlightChecks()
   at SourceTree.AppRoot.OnStartup(StartupEventArgs e)
   at SourceTree.App.OnStartup(StartupEventArgs e)