How to reproduce:
1. Open the attached "1052246.zip" project
2. Open "main" scene and "BugRepro" script
3. Add a breakpoint in line 11
4. Attach the debugger to Unity Editor
5. Press the Play button in Unity

Expected result: Unity continues to run
Actual result: when the breakpoint is hit, Unity closes

Reproducible with - 2017.1.0a1, 2017.1.4p2, 2017.2.3p2, 2017.4.6f1, 2018.1.7f1, 2018.2.0b9, 2018.3.0a3

Notes:
- not related to a specific IDE. Reproducible with both Visual Studio and JetBrains Rider
- the crash does not occur if no additional characters are added to the string (to check this, uncomment line 10)

Fixed only in .NET 4.x