Retain timer when it is being checked to prevent it from being released ...

...and then referenced. Resolves issue #138

This is for issue #138

during enumeration, `[timer check]` is being called which can cause the timer to be cancelled, and thus removed from the dictionary, deallocating it. `timer` was then being referenced at the end of the loop, causing a crash. This resolves the issue by retaining `timer` while it's in the loop.