How to reproduce:
1. Open the "UNET" project
2. Open the "Main" scene
3. Build the project
4. Enter Play mode
5. Choose "LAN Host(H)"
6. Open the built project
7. Choose "LAN Client(C)"
8. In the Editor press space (message "Sending: True" should appear)
9. In the client press space to disconnect, the host will throw errors

Expected result: When the client disconnects Unity stops sending messages.
Actual result: When the client disconnects Unity still tries to send messages.

Reproducible with: 2017.3.0a1, 2017.1.0p1, 2017.1.0a1, 5.6.2p4, 5.6.0a4.
Regression introduced in: 5.6.0a4.
Not reproducible with: 5.6.0a3, 5.5.4p1, 5.4.5p1.