DataItem of GridViewRow return null when firing update event of gridview

In my web application, i would love user to do update text in gridview. Because my datasource is always changed, I decided to use dynamic gridview. Everything is working fine except the update event of gridview. When already input their data in a textbox, I cant find that control and Gridviewrow always return null.
I used String value = ((TextBox)row.findControl("txtGer")).text.
Please help everyone.

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