Bug fix in columnChooser in case of usage grouped headers

Thank @Luca7993 (https://github.com/Luca7993) for reporting the bug. See [the issue](#261 (comment)) for more detailes.

js/grid.jqueryui.js

@@ -226,7 +226,7 @@
 									},
 									j + 1 // start enumeration with the index
 								);
-								$(items[iColNotInTheGroup >= items.length ? items.length - 1 : iColNotInTheGroup - 1])
+								$(items[iColNotInTheGroup === undefined || iColNotInTheGroup >= items.length ? items.length - 1 : iColNotInTheGroup - 1])
 									.after(items[indexOfAddedItem]);
 								updateNewColOrder();
 							}