Steps to reproduce:

1. Start Unity Editor;
2. Create new project and scene;
3. Open Asset Store window (Window -> Asset Store);
4. Detach Asset Store window or move it on another display;
5. Close Unity Editor.

Actual result:
Unity crashes (see attached gif video). In the stacktrace you can see that the following call crashes:
#0 0x0000010240c2f3 in WebView_CUSTOM_DestroyWebView(MonoObject*)

Regression introduced in Unity 5.3.0b3.

Also reproduces on Windows 8.1.