I had the dev build installed and the error happened so i reinstalled and went back to the old oxide and now i cant get it booted because once it hits that error it just crashes the server and starts to reboot it until it hits the same error again.
Error only appeared once i wiped the server (regardless of which Oxide version im using)

NullReferenceException: Object reference not set to an instance of an object
An exception was thrown! (NullReferenceException: Object reference not set to an instance of an object)
  at BaseNpc.AggroClosestEnemy () [0x0000a] in <8a091efae75f40548a1d23ee4bff57de>:0

Merged post

update - removed oxide and server is running on vanilla at moment with no crashes (2 hours later)

Merged post

kk this is really weird - i renamed the oxide folder and redownloaded oxide from here
It created a new oxide folder (which is what i wanted) no plugins at all 
Started server and it crashed again with the same error as above.So its not a plugin error as no plugins installed.
Here is the oxide log

09:40 [Info] Loading Oxide Core v2.0.3867...
09:40 [Info] Loading extensions...
09:40 [Info] Latest compiler MD5: 5fbca0bf39b1c09ddf2fb3686b869242
09:40 [Info] Local compiler MD5: 5fbca0bf39b1c09ddf2fb3686b869242
09:40 [Info] Loaded extension CSharp v2.0.4000 by Oxide and Contributors
09:40 [Info] Loaded extension Discord v1.0.0 by PsychoTea & DylanSMR
09:40 [Info] Loaded extension MySql v2.0.3755 by Oxide and Contributors
09:40 [Info] Loaded extension Rust v2.0.4142 by Oxide and Contributors
09:40 [Info] Loaded extension SQLite v2.0.3754 by Oxide and Contributors
09:40 [Info] Loaded extension Unity v2.0.3756 by Oxide and Contributors
09:40 [Info] Using Covalence provider for game 'Rust'
09:40 [Info] Loading plugins...
09:40 [Info] Loaded plugin Rust v2.0.4142 by Oxide and Contributors
09:40 [Info] Loaded plugin Unity v2.0.3756 by Oxide and Contributors
09:41 [Info] IP address from external API: 82.21.14.231
09:44 [Error] An exception was thrown! (NullReferenceException: Object reference not set to an instance of an object)
  at BaseNpc.AggroClosestEnemy () [0x0000a] in <8a091efae75f40548a1d23ee4bff57de>:0 
09:45 [Info] Loading Oxide Core v2.0.3867...
09:45 [Info] Loading extensions...
09:45 [Info] Latest compiler MD5: 5fbca0bf39b1c09ddf2fb3686b869242
09:45 [Info] Local compiler MD5: 5fbca0bf39b1c09ddf2fb3686b869242
09:45 [Info] Loaded extension CSharp v2.0.4000 by Oxide and Contributors
09:45 [Info] Loaded extension Discord v1.0.0 by PsychoTea & DylanSMR
09:45 [Info] Loaded extension MySql v2.0.3755 by Oxide and Contributors
09:45 [Info] Loaded extension Rust v2.0.4142 by Oxide and Contributors
09:45 [Info] Loaded extension SQLite v2.0.3754 by Oxide and Contributors
09:45 [Info] Loaded extension Unity v2.0.3756 by Oxide and Contributors
09:45 [Info] Using Covalence provider for game 'Rust'
09:45 [Info] Loading plugins...
09:45 [Info] Loaded plugin Rust v2.0.4142 by Oxide and Contributors
09:45 [Info] Loaded plugin Unity v2.0.3756 by Oxide and Contributors​

The CanBeTargeted hook for animals is disabled again for the time being.

thanks wulf