Error on startup preventing app from starting

When starting Source Tree I get this error message in the logs:

ERROR [2017-10-16 07:45:04,001] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-10-16 07:45:04,177] [1] [SourceTree.App] [OnStartup] - Failed to start
System.NullReferenceException: Object reference has not been specified to an instance of an object.
vid SourceTree.Notifications.NotificationsManager.SetOwner(NotificationDialogWindow notificationWindow)
vid SourceTree.Notifications.NotificationsManager.ShowNotificationDialog[T](NotificationDialogWindow notificationWindow, Tuple`2 customAction, VistaTaskDialogIcon icon)
vid 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)
vid SourceTree.Notifications.NotificationsManager.ShowNotificationDialogWithYesConfirmation(String title, String message, String details)
vid SourceTree.Configuration.WpfSpellCheckerPreFlightCheck.Run()
vid SourceTree.AppRoot.RunPreFlightChecks()
vid SourceTree.AppRoot.OnStartup(StartupEventArgs e)
vid SourceTree.App.OnStartup(StartupEventArgs e)