Incidental Contact rule

[Nevadaref]

A player with the ball may set a screen just as a player without the ball may set one. It doesn't happen often, but there is no rule against it.

What the rule is telling you is that if the screener does not have the ball the he/she may be displaced and it is still incidental contact. However, if the screener does have the ball, and he/she is displaced by the contact, then it is a foul on player who displaced him/her.