Fixing edge-case bug for history

[Windows81]

I occasionally get this error. It happens sporadically whenever I undo and I'm working on a fix for it. I think that it's a race-condition issue.

When it happens, History.Index is greater than #History.Stack. I believe that all the entries in History.Stack were valid when they were added in History.Add.

Players.VisualPlugin.Backpack.Building Tools.Core.History:31: attempt to index nil with 'Unapply'
    Stack Begin
    Script 'Players.VisualPlugin.Backpack.Building Tools.Core.History', Line 31 - function Undo
    Script 'Players.VisualPlugin.Backpack.Building Tools.Core', Line 144
    Script 'Players.VisualPlugin.Backpack.Building Tools.Libraries.SupportLibrary', Line 522
    Stack End