I've pushed fixes for both problems, the one you describe was because code skipped the vertex buffer creation in case of keyed particles without a valid target.
So this should be resolved, feel free however to comment again if issues persist...

It goes a bit further than that. After debugging the error, I found out that, when setting the physics' type to keyed, and proceeding to trigger the crash, the object's particle system somehow loses it's psys->particles reference, which in turn makes it return a psys->totpart of 0, which makes the object become an "invalid target", which makes the gpu_batch module thingie crash because verts is 0. I've been trying to find out why the particle reference is clearing its psys->particles.
Maybe there's something that's missing, something i don't understand about blender's pipeline.

Can confirm, problem seems to be twofold:

• under Relations, the Target Object as well as System counter should still be active when you dont have a valid particle target specified [ after all, this is how you would pick a valid one ;) ]
• crash should not happen

Dare setting this to High prio, since this is just a broken feature / regression...

Can confirm, checking...
probably caused by rB1342d1879e12

Looks like instances (in the instance collection) are not shown (whereas "regular" objects are).
Actually their empties are drawn in the viewport, but their meshes are not.

Can confirm, 2.79 also crashed here...

Can confirm on first sight, checking...

In T69659#772095, @Philipp Oeser (lichtwerk) wrote:

Can confirm this. (was not the case in 2.79)

Can confirm this. (was not the case in 2.79)

@Sybren A. Stüvel (sybren): maybe you could check on this?

I hear you :)
Will continue looking at it (as time permits), I also think we should at least iron out the worst bugs 2.8 has compared to 2.79...
[ squashed one yesterday :) rBe52ad1835a6a ]

(1) already asserts earlier now:
BLI_assert failed: /blender/draw/intern/draw_cache_impl_particles.c:1519, particle_batch_cache_ensure_edit_pos_and_seg(), at '!"Hairs are not in edit mode!"'
(Just dropping this here because I have seen in reports elsewhere - hopefully this will make it easier to track down the issue...)

Yes. Sorry about that - the video was a fast load before I was off to my office and I probably screwed it up somehow. It doesn't play here either. Can fix it after I get back to my studio tonight (four hours from now, five allowing for domestic fiddle-faddle...)

I will check T65038.