Fatal error: System.NullReferenceException: Object reference not set to an instance of an object.

While waiting 10 minutes for SoureTree to respond to a simple git push command, it threw the following error:

2015-07-20T10:17:21: 'git log' failed with code 128: fatal: Not a git repository (or any of the parent directories): .git
(C:\GitRepos\cameo-web)
2015-07-20T10:17:30: 'current revision' failed with code 128: fatal: Not a git repository (or any of the parent directories): .git
(C:\GitRepos\cameo-web)
2015-07-21T12:54:45: SingleInstance called with arguments: System.Collections.Generic.List`1[System.String]
2015-07-21T13:16:56: Fatal error:
System.NullReferenceException: Object reference not set to an instance of an object.

2015-07-21T13:16:56: Stack trace:
at SourceTree.ViewModel.CommitAndFileStatusViewModel.SubmoduleDialogCompleted(RepoDialogViewModel repoDlg, Int32 returnCode) in C:\projects\bitbucket.org\atlassian\sourcetreewin\ViewModel\CommitAndFileStatusViewModel.cs:line 908
at SourceTree.ViewModel.SubmoduleCheckDialogViewModel.<RefreshSubmodules>d__42.MoveNext() in C:\projects\bitbucket.org\atlassian\sourcetreewin\ViewModel\SubmoduleCheckDialogViewModel.cs:line 278
— End of stack trace from previous location where exception was thrown —
at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__0(Object state)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)