Program crashes during start-up and unable to reinstall it

SourceTree worked for me but today I'm not able to run it. If I look at Task Manager I can see, that program is terminating about 2 seconds after start. None window or error message is displayed.

I tried to reinstall program. I was able to uninstall it without any errors but when I try to run installation program, it crashes few seconds after start. I checked %AppData% folder and found error message in a log file. Don't know how to interpret it.

I do not installed any new software lately. Only some automatic updates from Microsoft.

I creaetd ZIP file with all files from PROFILE\AppData\Atlasian folder but can't attach it because "JIRA could not attach the file as there was a missing token."

Log file contents:
ERROR [2017-09-25 22:07:02,239] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-09-25 22:07:02,398] [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)}}