App error: Path cannot be the empty string or all whitespace. Parameter name: path

[thomas694]

Sometimes users let the app report an error that occurred, but it's unclear which steps caused it.
We have the error message, the stack trace and need to find the root cause...
Any help is welcome.

ExceptionMessage: Path cannot be the empty string or all whitespace.
Parameter name: path
ExceptionType: System.ArgumentException
Callstack:

at System.IO.Directory.GetParent(String path)
at TumblThree.Applications.ViewModels.SettingsViewModel.SaveSettings() in C:\projects\Tumblthree\src\TumblThree\TumblThree.Applications\ViewModels\SettingsViewModel.cs:line 1651
at TumblThree.Applications.ViewModels.SettingsViewModel.d__531.MoveNext() in C:\projects\Tumblthree\src\TumblThree\TumblThree.Applications\ViewModels\SettingsViewModel.cs:line 1477
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Waf.Applications.AsyncDelegateCommand.d__18.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

TumblThreeVersion: 2.14.2.457 (64 bit)