Timers are a handle type so the numeric value of 0 makes no sense and would be a type mismatch if you try to assign it to such a field.
If you want to null/nil it then you might need custom script as it does not look like GUI has "no timer" as an option. If you want to set its timeout to 0 then you need to start it with a 0 timeout. If you want to destroy the timer object then you need to use the appropriate DestroyTimer function, possibly with custom script, and should pause any running timer before destruction.