How to spawn enemies like clicker games

I'm having trouble on how to code random enemy spawns like in clicker games. I've tried to run a Destroy(this.gameObject); and then call the next GameObject, but in the update function that seemed to call back an error as its trying to call upon a GameObject that's already null. I tried doing a if (myObject.activeInHierarchy == false) { myObject2.SetActive(true); } But that seemed to be about the same as it deactivates object 1 and activates 2, but then it keeps running the script and crashes before I can set my next 100 GameObjects true. I just need help figuring out something that would spawn a random GameObject based on the length of how many there are, and then once that enemy either is inactivated or destroyed, call upon another random enemy.