There are different ways to solve this issue.
1. You have a
DataSet
that you use as the data source for the data grid
In this case you can change the behavior of the column to return an empty string instead of DbNull
2. Change the code used to save the data
You are not showing your code but this an example of how to do it
Assuming
Reason
is the name of your column.
string s = (dg["Reason", i] == null) ? "" : dg["Reason", i].Value.ToString());
da.Parameters("@Reason", s);