Please update the easyui to version 1.4.2. If you are using version 1.4.1, please refer to the code below to solve your issue.
$('#dg').datagrid({
onBeginEdit: function(index,row){
var ed = $(this).datagrid('getEditor',{index:index,field:'productid'});
$(ed.target).combotree({
url: '...',
value: row.productid
});
},
onEndEdit: function(index,row){
var ed = $(this).datagrid('getEditor',{index:index,field:'productid'});
row.productname = $(ed.target).combotree('getText');
}
})