When WebView windows are closed (by entering resetting the layout or closing them manually) Unity crashes.

Steps to reproduce:
1) Create a new project.
2) Go to Window > Services.
3) Quickly right-click on the Services tab > Close.

Unity crashes in UnityEditor.WebView.DestroyWebView(). The same happens with Asset Store tab. If the contents of the tab finish loading, the crash does not occur.

Reproduced on:
5.6.0b1, 5.6.0a6

Not reproduced:
5.5.0f3, 5.4.3p3

OS tested:
macOS Sierra 10.12.1 - reproduced
macOS Sierra 10.12 - reproduced
OS X 10.11.6 - not reproduced
Windows 10 - not reproduced