Undo / redo validity condition seems wrong in the presence of stages

[ezyang]

Currently it says:

boardgame.io/src/master/master.ts

Lines 242 to 253 in 60b32e5

	// Only allow UNDO / REDO if there is exactly one player
	// that can make moves right now and the person doing the
	// action is that player.
	if (action.type == UNDO || action.type == REDO) {
	if (
	state.ctx.currentPlayer !== playerID ||
	state.ctx.activePlayers !== null
	) {
	logging.error(`playerID=[${playerID}] cannot undo / redo right now`);
	return;
	}
	}

But activePlayers can be non-null and there is only one valid player. For example:

activePlayers: {0: "selectColor"}

It seems to me that undo should be allowed in this case.