Threat table exception

Summary

At some point, one or more mobs create a exception loop with Threat Tables and the target being null. MythicMobs version 5.7.2-SNAPSHOT-c4bdb8f7

Steps to reproduce

I was not able to replicate it by myself, but it happens every single day at least once.

Current behavior

Does not affect anything to my knowledge. This exception does not go away if the plugin is reloaded via /mm reload or/and killing all mythicmobs.

Debug log snippet

https://mclo.gs/8bUe2af

Proposed fixes

Check if the target is null when ticking a threat table. This issue is really close to issue #468 (closed)