[VB6] - strange bug that can block my VB6:(

i don't know why, but these code works fine in a module(Standard exe project):

Code:

---

.............
MsgBox Screen.ActiveForm.Controls(1).Name
............

---

error message:
"run-time error '91':
object variable or with block variable not set"
but using these module in Activex project.... my vb6 is blocked. and for close it i must use ctrl+alt+del(combined keys).
it's strange these code is only for read and i have 100% sure that is correct.
can anyone tell me why these stupid error?