Steps to reproduce:

1. Create a new project.
2. Go to Window -> Layouts -> Save Layout...
3. Type anything with a '/' in it, (ex. "Lay/out") and try to save it.
4. After the error pops up, press cancel.
5. After doing the given steps, try to repeat the second step.

As a result, Unity crashes.

Reproduced on: 5.5.0b1, 5.4.0p4, 5.4.0p3, 5.4.0p2, 5.4.0f3,

Reproducible on earlier versions of 5.X with a Unity browser window open (Asset Store, Services, etc).