Cached SortedColumnId causing UnknownValueException with Dynamci columns

Tapestry crashes with an UnknownValueException if a Grid is sorted by a column not existing in the PropertyModel of the Grid anymore. 

This can happen if the columns of the grid are dynamic and can change at runtime (e.g. using the component paramter "include" and include columns based on further conditions).

A check prior getting the sort constraints of the sort model could prevent this issue.