MVC Scaffolding error: "Value cannot be null. Parameter name: source"

Your problem is the following:

You assign this property within the Controller:

ViewBag.PossibleCategory = context.Categories;

Then, in your View you try to read this dynamic ViewBag property:

ViewBag.PossibleCategories

Can you see the error? You're giving different names... You do not get compile time checking because ViewBag uses the new C# 4 dynamic type. ViewBag.PossibleCategories will only be resolved at runtime. As there's no ViewBag property that matches ViewBag.PossibleCategories you get this error: Value cannot be null. Parameter name: source

To solve this just do this:

ViewBag.PossibleCategories = context.Categories;