Help with 3D Platformer Tutorial

I am having trouble with the robots. So far I have the robot out in the scene and it will attack you if you come near it. But what is happening now is, you kill the robot then you get an error and the game freezes. So I went on to the next step in the tutorial and that was,

Create an empty GameObject at the top level of the Hierarchy Pane. Rename it CopperSpawn. Drag the EnemyRespawn script onto the object. Position our CopperSpawn object somewhere where youd like to see a robot appear. (You should see a small robot icon displayed as a gizmo, as well as a sphere to show the spawn range. These are drawn by the EnemyRespawn script.) Adjust the objects settings as shown.

so I did that and what I get was a robot that just sat there. And the other robot still attacking and then freezing as I said above. Any advice?? Thank you.