Steps to reproduce:
1. Create a new Unity project
2. Change the Scripting Runtime Version to .NET 4.6 Equivalent (Project Setting -> Player -> Other Settings)
3. Create a new Canvas and add a new Text GameObject as its child
4. Paste "??" into the Text Component text field
5. With back space deleted it.

Results: Unity crashes when Unicode character is deleted

Note: Doesn't reproduce when using .NET 3.5. Editor crashes just with those characters which code contains 5 symbols (U+1F5D8).

Reproduced with: 2017.3.0a3, 2017.2.0b6, 2017.1.0p3