Seems to be a duplicate of SRCTREEWIN-7935. (Sorry, I had been looking in the SRCTREE project for potential duplicates.)

Recently, Source Tree crashes on startup afer briefly showing a logo. I tried uninstalling and reinstalling it, but it does not solve the problem. The end of the sourcetree.log file reads:

ERROR [2017-09-21 13:03:05,935] [1] [SourceTree.App] [.ctor] - finish EnsureSquirrelExecutionStubIsCopied
ERROR [2017-09-21 13:03:06,188] [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)

The SquirrelSetup.log file reads:

2017-09-21 13:08:47> Program: Starting Squirrel Updater: --install .
2017-09-21 13:08:47> Program: Starting install, writing to C:\Users\Felipe Gerhard\AppData\Local\SquirrelTemp
2017-09-21 13:08:47> Program: About to install to: C:\Users\Felipe Gerhard\AppData\Local\SourceTree
2017-09-21 13:08:48> CheckForUpdateImpl: Couldn't write out staging user ID, this user probably shouldn't get beta anything: System.IO.DirectoryNotFoundException: Could not find a part of the path 'C:\Users\Felipe Gerhard\AppData\Local\SourceTree\packages\.betaId'.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   at System.IO.StreamWriter.CreateFile(String path, Boolean append, Boolean checkHost)
   at System.IO.StreamWriter..ctor(String path, Boolean append, Encoding encoding, Int32 bufferSize, Boolean checkHost)
   at System.IO.File.InternalWriteAllText(String path, String contents, Encoding encoding, Boolean checkHost)
   at System.IO.File.WriteAllText(String path, String contents, Encoding encoding)
   at Squirrel.UpdateManager.CheckForUpdateImpl.getOrCreateStagedUserId()
2017-09-21 13:08:48> CheckForUpdateImpl: Failed to load local releases, starting from scratch: System.IO.DirectoryNotFoundException: Could not find a part of the path 'C:\Users\Felipe Gerhard\AppData\Local\SourceTree\packages\RELEASES'.
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share)
   at Squirrel.Utility.LoadLocalReleases(String localReleaseFile)
   at Squirrel.UpdateManager.CheckForUpdateImpl.<CheckForUpdate>d__2.MoveNext()
2017-09-21 13:08:48> CheckForUpdateImpl: Reading RELEASES file from C:\Users\Felipe Gerhard\AppData\Local\SquirrelTemp
2017-09-21 13:08:48> CheckForUpdateImpl: First run or local directory is corrupt, starting from scratch
2017-09-21 13:08:48> ApplyReleasesImpl: Writing files to app directory: C:\Users\Felipe Gerhard\AppData\Local\SourceTree\app-2.3.1
2017-09-21 13:08:52> LogHost: Rigging execution stub for SourceTree_ExecutionStub.exe to C:\Users\Felipe Gerhard\AppData\Local\SourceTree\SourceTree.exe
2017-09-21 13:08:54> ApplyReleasesImpl: Squirrel Enabled Apps: [C:\Users\Felipe Gerhard\AppData\Local\SourceTree\app-2.3.1\SourceTree.exe]
2017-09-21 13:08:56> ApplyReleasesImpl: Starting fixPinnedExecutables
(...)
2017-09-21 13:08:56> ApplyReleasesImpl: Fixing up tray icons
2017-09-21 13:08:56> ApplyReleasesImpl: cleanDeadVersions: for version 2.3.1
2017-09-21 13:08:56> ApplyReleasesImpl: cleanDeadVersions: exclude folder app-2.3.1