Hi, i’ve had similar problems after doing the zombie FPS course from gamedevtv on Udemy. Similar issue I think to br222. When my last shot that killed the zombie happens, the game broke with the same error. The navMeshAgent.enabled is set to = false which is good to stop the zombie continuing to move but there were other calls continuing in EngageTarget() then ChaseTarget() and that kind of thing. Was actually a really easy fix and all i had to do was add a line to say isProvoked = false as then EngageTarget() stops being called.
In my case, I was instantiating a prefab that already had a NavMeshAgent component on it and getting the error even though I was spawning the prefab in the right place. This must have been causing something to initialize in the NavMeshAgent before it was actually placed in the scene, because removing the NavMeshAgent component from my prefab and instead creating the NavMeshAgent in the Start() method of the game object resolved the error for me.