Steps to repro:

1. Open attached project and 'test' scene;
2. In the project folder open 'test' script;
3. Set the breakpoint on line 9;
4. Attach debugging tool to Unity Editor;
5. Launch the scene;
6. In the code editor click "Step Over" button (one or several times in a row).

Actual result:
Editor crashes (see attached gif video).

Last calls from the stacktrace:
#0 0x000001022183bf in InitPlayerLoopCallbacks()::EarlyUpdateUpdateMainGameViewRectstruct::Forward()
#1 0x00000101a9f53e in PlayerLoop()
#2 0x000001025e5765 in Application::UpdateScene(bool)
#3 0x000001025e5e5d in Application::EnterPlayMode(bool)
#4 0x000001025e253c in Application::SetIsPlaying(bool)
#5 0x000001025e1e4d in Application::TickTimer()

Regression introduced in Unity 5.5.0b5.

Expected result:
Editor should not crash when moving to the next line of code in debug tool.

Notes:
- Reproducible in 5.5.0b5;
- Not reproducible in 5.5.0b4;
- Reproducible on both OSX 10.11.6 (MonoDevelop 5.9.6) and Windows 10 (Visual Studio 2015 v12.023107.0);
- Also reproducible when stopping the Debug tool (by clicking Stop button).