The arduino is crashing because you have not isolated the motor power from the arduino power. You need to forget whatever plan you had and follow my advice. You CANNOT , REPEAT CANNOT have the motor power and the arduino power tied together for reasons I won't even go into right now. You will just have to take my word for it. Use the 12V bench supply for the motor only .
Is it variable or fixed 12V ? Use the USB cable to power the arduino. If you can't do that for some reason then buy a 9V wall -wart
and plug that into the arduino ext.pwr dc barrel jack. You can order one of these :
https://solarbotics.com/product/bholdaa-5cell-p/ .
Do not post any more about this until you change your power setup. There is no point. Your arduino must have a separate power supply. It's as simple as that. There is no mystery to why you are having a problem. You are not the first to try that and you won't be the last. Put a multimeter on the 12V bench supply BEFORE you attempt another test. When you do the test , watch the meter as soon as you start the program.
Any recommendations?
Yes. Don't do that again.