SourceTree fails to start

SourceTree fails to start even if previous versions are removed and cleaned from the system.

App fails to start because in SourceTree.App.OnStartup a System.NullReferenceException is thrown.

ERROR [2017-10-30 12:30:11,586] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-10-30 12:30:11,773] [1] [SourceTree.App] [OnStartup] - Failed to start
System.NullReferenceException:
   SourceTree.Notifications.NotificationsManager.SetOwner(NotificationDialogWindow notificationWindow)
   SourceTree.Notifications.NotificationsManager.ShowNotificationDialog[T](NotificationDialogWindow notificationWindow, Tuple`2 customAction, VistaTaskDialogIcon icon)
   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)
   SourceTree.Notifications.NotificationsManager.ShowNotificationDialogWithYesConfirmation(String title, String message, String details)
   SourceTree.Configuration.WpfSpellCheckerPreFlightCheck.Run()
   SourceTree.AppRoot.RunPreFlightChecks()
   SourceTree.AppRoot.OnStartup(StartupEventArgs e)
   SourceTree.App.OnStartup(StartupEventArgs e)