Try using the latest version of easyui, otherwise rewrite the 'getChecked' method as below:
$.extend($.fn.datagrid.methods, {
getChecked: function(jq){
var rr = [];
var rows = jq.datagrid('getRows');
jq.datagrid('getPanel').find('div.datagrid-cell-check input:checked').each(function(){
var index = $(this).parents('tr:first').attr('datagrid-row-index');
rr.push(rows[index]);
});
return rr;
}
});