To reproduce:
1) Download the project attached by the user
2) Setup the 2 models in the project as follows:
1. set rig of human.fbx : avatar definition to Create from this model
2. set rig of blacksmith_01 : avatar definition = Copy from Other Avatar, use HumanAvatar defined by human.fbx
3) Put Blacksmith_01 prefab in a scene
4) Add an animator component and set the avatar to the one from human.fbx
5) Open animator component's settings window and select "optimize transform hierarchy"
Result: Editor freezes and Unity starts leaking memory

Reproduced on: 2018.3.0b1, 2018.3.0a6
Regression introduced with: 2018.3.0a6
Not reproducible on: 2018.3.0a5, 2018.2.6f1