The app doesn't start at all.  Even after a clean reinstall.

In the AppData "Atlassian" folder, I can see a sourcetree.log file with the following content:

ERROR [2017-10-06 14:20:44,063] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-10-06 14:20:44,198] [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)