Operations is not valid due to the current state of the object

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I'm just trying to make a "Kill Player" ability for admins, I just want to make so a list of players shows up (With their names) and you can click on the player and kill them. This means finding not only the players in the room but the objects they control & the objects they control that have health. Idk if I did it wrong (it seems like it's more complicated than it needs to be) who knows, all I know is whenever I try to run it I get the error: "Operations is not valid due to the current state of the object".

The Code:
http://pastecode.org/index.php/view/28638672