How to implement a respawn/lives system in Space Shooter (the tutorial game)?

Hello I've been following along with the excellent tutorial for Space Shooter. I'm mostly finished with the game but I wanted to try to add more things to it to give it a much more finished feeling! This includes a lives system, power-ups, scrolling background, menu, different looking levels, etc. etc.

I'm trying to implement a lives system right now. Basically, having a limited amount of lives and when it reaches player.lives == 0, the GameOver() method is called.

I've tried a couple of different things, including creating an IEnumerator method for respawning in GameController, and that didn't seem to work too well since I try to Instantiate the game object after destroying it and Unity tells me there's no Object associated with the variable anymore... so I am a little bit confused.

Anyone have any tips for implementing this?

Thanks!