>> I have Squeak6.0alpha-17945-32bit.image and don't see this.
>> I update now for see some new introduce the bug
...
> 
> No, right now update to 17970 and do not show this

I just reproduced it again.

I downloaded the Squeak6.0alpha-17967-32bit-201712221331-Windows archive 
from http://files.squeak.org/6.0alpha/ and used the VM in that archive 
to open Squeak6.0alpha-17967-32bit.image

Then I alt-clicked on the welcome window and get its halo, clicked on 
the grey handle, did "explore morph", then from the explorer that opens 
I right-clicked on the first line and did "browse class". Got the DNU.

Stef