Firstly, it's a better game plan to identify theObject specifically. Otherwise, as soon as you put an on clicked hander for another object it will diligently obey the same handler code. So:
on clicked theObject
set n to name of theObject
if (n is "my_button") then
tell window of theObject
try
set theRate to contents of text field "rate"
set theAmount to contents of text field "amount" as number
set contents of text field "total" to theRate * theAmount
(* ==== How do I call a handler In Xcode?? ==== *)
my Handler()
on error
set contents of text field "total" to 0
end try
end tell
end if
end clicked
on Handler()
display dialog "It's me!"
end Handler