你检查过row.Cells的值[3] .Text ??

Have you checked the value of row.Cells[3].Text??

亲爱的请先填写你的下拉菜单然后设置下拉值。由于下拉列表为空，它给出错误

Dear please fistly fill your drop down then set the dropdown value. it is giving error because of dropdown list is null