You would use an interruptible region represented by a dashed line box:
The timer interrupt appears independently and intrerrupts the current action. Auto save is executed and comes back with Fill form. Resuming Fill form need a bit of thought since usually you have some entry code which must not be executed in case of a continuation. That might be a bit more tricky as you would likely need a mutex for that.
UML 2.5 has a detailed description in chap. 15.6.3.2 Interruptible Activity Regions on pp. 405.
Just a word about your approach. The save is only executed when the form is closed. So if it takes longer than 5 minutes you end up in your form again which is likely not desired :-)
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