.load() should check if null or undefined is returned and throw an error

The function is always expected to return a Customize-instance, but at the moment, it just returns the return value of the parameter-function.

If that function return "null", it will be passed and following commands wil fail (with non-intuitive error-messages) 

It should also make sure that a Customize-instance is returned and not (e.g. a string)