Your Bot_AA_Character is null in one of the scenarios that exists during the game lifetime, whether at startup, at the end of the game or another such case.
It is good practice to always perform “Is Valid” checks on objects that you are referencing, this will prevent any “Accessed None” errors you would otherwise encounter.
To stop this error from showing, simply add a node after your cast that checks if the Bot_AA_Character is valid.
Hope this helps 