NullReferenceException on commit

SourceTree was crashed with NullReferenceException on commit.

1. select a unstaged file.
2. right click -> select "Commit"
3. crashes with following logs.

2014-10-09T10:50:43: Fatal error:
System.NullReferenceException: Object reference not set to an instance of an object.

2014-10-09T10:50:43: Stack trace:
at SourceTree.ViewModel.RepoTabViewModel.CommitSelected()
at SourceTree.ViewModel.Utils.CommandSink.ExecuteCommand(ICommand command, Object parameter, Boolean& handled)
at SourceTree.ViewModel.Utils.CommandSinkBinding.<set_CommandSink>b__1(Object s, ExecutedRoutedEventArgs e)
at System.Windows.Input.CommandBinding.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.Input.CommandManager.ExecuteCommandBinding(Object sender, ExecutedRoutedEventArgs e, CommandBinding commandBinding)
at System.Windows.Input.CommandManager.FindCommandBinding(CommandBindingCollection commandBindings, Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
at System.Windows.Input.CommandManager.FindCommandBinding(Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
at System.Windows.Input.CommandManager.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.Input.RoutedCommand.ExecuteImpl(Object parameter, IInputElement target, Boolean userInitiated)
at System.Windows.Controls.MenuItem.InvokeClickAfterRender(Object arg)
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)