so basically i am practicing making custom mobs that target specific mobs.

All the mobs do is spawn in and not target anything (well except for me if i change to survival).

It does not give any syntax errors, but it does give a runtime error:

"Silently" catching entity tracking error.
net.minecraft.server.v1_8_R3.ReportedException: Adding entity to track
Caused by: java.lang.IllegalStateException: Entity is already tracked!