Steps to reproduce:
1. Open Project "NSMenuItem_Bug"
2. Click on the red node labeled "Apartment" in the "Dialogue" Window
3. In the Inspector Window select the first drop-down Menu under "Links To" item, chose "Fight/Alley"
4. Selecting the second drop-down Menu will freeze the Editor (there will be assertion failure errors

Expected result: Opening a drop-down menu is fluent and doesn't freeze Unity Editor
Actual result: Opening a drop-down menu freezes Unity Editor (Assertion failures in the Editor.log)

Reproducible with: 2018.3.11f1, 2019.1.0b9, 2019.2.0a10
Not reproducible with: 2017.4.24f1

Note: Issue only occurs on MacOS