Simply closing mac's display when an external monitor is still on worked for me.

My setup: MacBook Pro Sierra 10.12.6 External monitor connected through HDMI External mouse and keyboard connected to the monitor

Simply closing mac's display when an external monitor is still on worked for me.

My setup:

• MacBook Pro Sierra 10.12.6
• External monitor connected through HDMI & monitor USB cable
• External mouse and keyboard connected to the monitor