With kde4 everything work as expected. After update to plasma 5.4 and kscreen 5.4, I'm unable to use external monitor correctly.

I've a laptop with intel i7 (sandybridge, first generation) and I'm using the internal intel videocard.
When I attached an external monitor, with the fn key+f7 I was able to change monitor disposition (right, cloned, left, only external monitor).
Now (plasma 5.4) this functionality seems to work, but when I detach the external monitor, the screen of the laptop stay black. If I press fn+f7, the screen reappear, left plasma panel disappear (I use a left side panel instead of standard horizontal panel) and after about five second plasma and kwin crash. 
After reboot, plasma boot up without left panel and the only way to give it back is to reconnect external monitor, change position from left side of monitor to bottom side, detach monitor,  press fn+f7 and wait for plasma and kwin crash .

Reproducible: Always