Editor Crashes while playing if Prefab is not assigned to SpawningPool in the attached project.

Steps to Reproduce:
1. Create a new project and import attached package.
2. Create a GameObject and attach the Spawner script to it.
3. Create another GameObject and attach SpawningPool script to it, assign it to Spawner.
4. Play the scene and observe crash.

Reproduced On: 2018.1.0a6, 2018.1.0a7, 2018.1.0b4

Working Fine On: 2017.3.0f3, 2018.1.0a3, 2018.1.0a5

Notes:
- The crash appears only when Prefab is not assigned to SpawningPool script.
- Appears always
- Appears on both Mac and Windows