onDeactivate event.

I've created two forms and put a control on the first one.
The second form has a variable (from the same class of the control)
that points this control.
in onDeactivate event of the second form
I made some changes with the control (using the variable
that pointed to it). I got an exception for this thing when
I was moving from the second form to the first by clicking
on the control. This happened to me with TChart control,
while I was deleting point in series,  
but I'm wondering if this act is basically wrong to do with
controls in Delphi ?
If anyone has an idea what causes this problem, please reply this letter.
Lots of thanks, Gidi.