Using Dropdownlist in Gridview for editing purposes

I have a drop down list in a gridview edit template column. Two questions:

When I go into DataBindings, and attempt to select a field for a data
binding, it is greyed out, so I have created a custom binding to a field
from the datasource Why is this greyed out, what conditions have to be met?
I have other gridviews that it is not greyed out and I do not see the
difference.

The second problem is when a record related tothat field is null, I get a
"System.ArgumentOutOfRangeException: 'DropDownList1' has a SelectedValue
which is invalid because it does not exist in the list of items."

I tried adding a blank item in the list but still get the error. How can I
fix this?



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