Yeah, the problem is that the newly spawned unit can't continue to occupy the city center tile but also can't move to any other tile, since they are all occupied. For now, all you can do is delete the newly spawned unit (at least you get gold). Hopefully this will be patched to prevent the unit production from being completed (or some similar solution), although here it appears that you could have moved the archer from the tile SW of the city to the tile across the river or further south between the two mountains to make room for the warrior.