reset method
void
reset()
Resets the timer.
Sets the timer to its original duration and rearms it if it was already expired (so it can be started again).
Implementation
void reset() {
if (isCancelled) return;
_stopwatch = clock.stopwatch();
if (isActive) {
_timer!.cancel(); // it has to be non-null if it's active
_startTimer();
}
}