How to fix "unfortunately , the process com.android.systemui has stopped" error?
I have been experiencing this as well on the same build/phone.  A user on this thread claims that M5 may be much less error prone.  

I haven't reverted to M5 yet myself.  When this error loop happens to me, I hold down the power button and try to select "reboot" between the times when I hit ok in the "Unfortunately... " box.