I had this issue without knowing I had it (or that it was called a cache bug) 
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In the RPG Combat - Scene Management, where we make the Portals I added a simple “In/Out” animation where I move the player from the spawn point to a destination point (Animated Warp Portal Transition ).
This involved getting the player and navmesh component multiple times, so I thought it would be “Smart” to get it in Awake . Turns out you get a nice “Null exception” since the cached “player” I was referencing was deleted once he stepped in the portal. Went back and simply referenced the player in both functions, and it fixed the issue.