Referencing child controls on a formview

[Neil]

I want to be able to set the enabled status of certain controls on the
edititem template of a formview control based on whether a check box is
checked or not (or the value coming from the database as stored by the
sqldatasource) .

I've tried code such as:

CheckBox cb =
(CheckBox)FormView1.Row.FindControl("SysDefMethod" );
Button ub = (Button)FormView1.Row.FindControl("UpdateButton");
if (cb.Checked == true)
{
ub.Enabled = false;
}

in various events (formview databound & databinding, sqldatasource
selected) but always get an "Object reference not set to an instance of
an object." exception when the "if (cb.Checked == true)" line is
executed.

The item to be displayed in the formview is chosen from a gridview. I
have no problem getting the data displayed or updated, its just that I
now want to restrict the fields that are available to the user based on
the value of SysDefMethod (it's a bit field in the database).
Thanks in advance

[Neil]

Does no-one have a solution for this.

I can't see anything mentioned in books or on MSDN as to how I can do
this.

Neil