[kylin-display-switch]Unplug the external monitor, laptop becomes black screen.

[kyt]

[Machine]
Dell Vostro 3900
  [Version]
1804 final 20180427.3 enhanced
  [Details]
 [kylin-display-switch] Unplug the external monitor, laptop becomes black screen.
【显示切换】显示切换时，拔下显示器，笔记本上黑屏无法唤醒，切换tty1再切换至tty7，输入密码进入桌面，桌面和窗口管理器异常。
  [Test Case]
1. Connect two monitors
2. press super+P to switch among four display modes
3. Unplug the external monitor
Expected Result: Passed
Actual Result:
laptop becomes black screen. Press Ctrl+Alt+F1 to enter tty1 and then press Ctrl+Alt+F7. Enter the password in the greeter. The desktop and the window manager disappear.

[Karambir Singh Nain (karambir)]

This happens even with one monitor on Thinkpad T470.

I have setup my external monitor to be only display. If I simply disconnect the monitor, the laptop screen goes blank(black). If my monitor loses power supply first(if I switch it off) and then unplug it, the laptop screen picks up being main display.