Quick update: I chose Google as Assistant in step 2 of what Vass66 recommended above and it worked well without crashing or anything even after exiting menu and closing open apps.
Maybe the error and crash you told @Vass66 was fixed in this 2.16?
Phone freezes after long press on Home Button, even though it should launch Google Now. I tried to config it in Settings-> Buttons and did, but same thing happens.
Mi5S
Is there something to manually fix this thing? I love this ROM