You don't have to choose a target for Razorgrass Invoker's last ability. However, if you do and the target creature is an illegal target as the ability tries to resolve, it won't resolve and none of its effects will happen. Razorgrass Invoker won't get +3/+3. (Razorgrass cuts both ways, apparently.)
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.