Flash Trivia Game Timer

[COLOR=#000000][FONT=arial]Hi their!
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[COLOR=#000000][FONT=arial]I am attempting to make a Flash AS3 trivia game using dynamic text fields etc. I’m struggling with the timer. I have attached the files to see if anyone could help me figure this problem out. Please can someone help me as soon as possible, it would be much appreciated.

Error message:

TypeError: Error #1009: Cannot access a property or method of a null
object reference.
at SmartGame_fla::MainTimeline/countdown()
[SmartGame_fla.MainTimeline::frame20:14]
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

I am new to Flash so any help would be fantastic! Thank you in advance.

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