The idler gear (yes you are right on the spelling!) has nowhere to go. If you put it in correctly then it should be trapped between the axle and the chassis. It cannot fall out even if you remove the motor.
Have you double-checked your instructions?
In any case, I don’t see how anyone could put the idler gear wrongly, it just won’t fit in * your car would not work.
no no, i put it the right way, but the idler gear slides out even when everythings put together, i flip it upside down, and i see it moving back and forth against the wheel gear and the chassis, it wont just stay in place