Assertion in method move due to Stand Firm skill

[Drefsante]

Following assertion occurs in method 'move' of game.py:
assert self.get_player_at(position) is None or (type(piece) == Player and piece.state.in_air)

In this situation:

The left red player is pushing the blue one: we have to select another player to push -> we select the Treeman who uses the skill Stand Firm
-> we get the exception