Any cop who confused their gun for their taser has zero business being a cop.
I know you are gone but I agree.
In the heat of the moment, life and death type crap, your brain doesn't work as it normally would. You react and muscle memory is paramount. If you haven't trained properly and if you haven't developed the correct muscle memory, this type of mistake is entirely predictable.
During an adrenaline dump, you will not notice the weight difference or the difference in texture between a gun and a taser.
As an FTO, we run drills based on different threats that address this exact issue. However, that's a starting point and the LEO's themselves must continue to train themselves periodically including pulling your taser, your firearm, dropping your magazine and loading another and clearing a gun jam (tap, tap, rack).