Hello folks, 
I am just checking in, because I was searching for a solution for aaaages. Now that I have it, I want to help everybody having the same issue, solve it as well.
Issue:
Whenever I went to “overview screen” in Gnome-Shell (Windows Key), and tied to move an open window, the system crashed with “Oh no! Something went wrong” and logged me out. Discarding unsaved stuff. 
I tracked it down with journal-ctl to:
No cursor theme available, please install a cursor theme
gnome-shell[9341]: Received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadCursor (invalid Cursor parameter)'.
(Details: serial 30266 error_code 6 request_code 95 (core protocol) minor_code 0)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the MUTTER_SYNC environment
variable to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the mtk_x_error() function.)
Many posts on the internet tried to connect this to xorg, wayland or any version or customization. All unrelated. 
What it is:
When moving/dragging a window the mouse cursor changes. Specifically it changes to dnd-none. If mutter doesn’t find this, it crashes. So it is a bug in mutter. I don’t know which, it is quite old. I found some issue in an old code repository but that moved. So… feel free to add the current bug in the comments and/or add similar reports!
Solution:
Make sure dnd-none exists. As I was using uncustomized, plain Adwaita icon theme, I don’t know why it was missing. Just do:
sudo ln -s /usr/share/icons/Adwaita/cursors/dnd-move /usr/share/icons/Adwaita/cursors/dnd-none
This creates a link for the dnd-none icon to the dnd-move icon, which I had.
That’s it. I saw this issue reported for a variety of distros. I myself use Gentoo. Others had this in Ubuntu. No clue, how a file goes missing. Anyhow, mutter should not expect any mouse cursor to be provided and otherwise crash this hard. I know better ways how to waste time! 
Cheers.
PS: It still scares the shit out of me, when I move the mouse and accidentally the overview screen zoomes out! I always expect to have lost my unsaved work! This is a real trauma! 