Repro steps:
- Open the project attached or download the package from here: https://oc.unity3d.com/index.php/s/qiSRFgzvoQiMKaS
- got TO \Low Poly Animated Dinosaurs\- Prefabs\Dinosaurs and try to drag some dino prefabs to the scene
- notice Unity will freeze
- sometimes the Editor will recover from the freeze after 2 minutes or so
- switching to "prefab view" repeatedly by double-clicking on prefabs also triggers this

Note! this cannot be replicated in 2018.3.0b1 because unity will not import the package at all
Replicated in Unity 2018.3.0a11
Some shorter freeze also occurred in 2018.3.0a2 and 2018.2.1f2 so not entirely sure if it's a regression..