[Known]second pokemon wont come out

[palmagius 0]

in the battle with fern before fighting the poison leader i knocked out his first mon (rhyhorn) and he wont bring out a second mon. the game simply says what will x do next and when i attack is keeps saying there is no target. every turn a error pops up from the start. the game continues after i press ok but the error pops up again the next turn. i tried using different mon's and moves but that wont work. if my mon faints first then i cant send out a second mon either. i play on version 18

 

(edit) i tried some more different things to get more info about this bug and i noticed the error also happens when fighting mon's in the wild outside the gym. but after beating those the game continues since its only 1 mon. the only thing these battles had in common is the field so the bug is caused by the field for some reason