fix moves preview

app/src/main/java/chess/controller/CommandExecutor.java

@@ -1,6 +1,7 @@
 package chess.controller;
 
 import chess.controller.Command.CommandResult;
+import chess.controller.commands.UndoCommand;
 import chess.model.Game;
 import chess.model.Game.GameStatus;
 
@@ -21,7 +22,7 @@ public class CommandExecutor {
 		CommandResult result = command.execute(this.game, this.outputSystem);
 
 		// non player commands are not supposed to return move result
-		assert result != CommandResult.Moved || command instanceof PlayerCommand;
+		assert result != CommandResult.Moved || command instanceof PlayerCommand || command instanceof UndoCommand;
 
 		processResult(command, result);