You can write code in the form with the timer just not when the form is open in form view. Why would you need a form to be open when you are writing code?
Googling "ms access why do timers affect the vba editor?" does seem to give any good answers as to why? In my opinion this is a bug. Apparently when the process thread is switched to run the timer it does return to the editor thread in the same state as in left.