I've had the same issue, but only on some textboxes in grids.
My sollution was to first click in the textbox (turns out that a border was above the textbox, when i clicked it then my TextBox appeared), then i could read the ITextBox object as text.
One of my example:
var myTextBox = rutin.ItemTextbox;
var myBorderThatIsBlocking = rutin.ItemBorder;
This helped me, perhaps it can help you aswell.