Description of problem:

gnome-shell segmentation faulted in meta_backend_get_monitor_manager at ../src/backends/meta-backend.c:1138 in mutter-40.0~beta-1.fc34.x86_64 when logging out of GNOME 40 beta on Wayland. The pointer priv was an inaccessible address possibly because the backend pointer was null as an argument to meta_backend_get_monitor_manager.

Core was generated by `/usr/bin/gnome-shell'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007f85a500f5cb in meta_backend_get_monitor_manager (backend=0x0) at ../src/backends/meta-backend.c:1138
1138      return priv->monitor_manager;
[Current thread is 1 (Thread 0x7f85623f7640 (LWP 21307))]

(gdb) bt
#0  0x00007f85a500f5cb in meta_backend_get_monitor_manager (backend=0x0)
    at ../src/backends/meta-backend.c:1138
#1  0x00007f85a510a83e in meta_is_stage_views_scaled () at ../src/backends/meta-backend.c:1507
#2  meta_seat_impl_filter_relative_motion.constprop.0
    (seat_impl=seat_impl@entry=0x55fce2e66b60, x=757.002502, y=463.662476, dx=dx@entry=0x7f85623f655c, dy=dy@entry=0x7f85623f6558, device=<optimized out>) at ../src/backends/native/meta-seat-impl.c:1147
#3  0x00007f85a50f985d in meta_seat_impl_notify_relative_motion_in_impl
    (seat_impl=0x55fce2e66b60, input_device=0x7f85340b10d0, time_us=6742284184, dx=<optimized out>, dy=<optimized out>, dx_unaccel=-1, dy_unaccel=0) at ../src/backends/native/meta-seat-impl.c:568
#4  0x00007f85a51065e4 in process_device_event (event=0x7f8534034cf0, seat_impl=0x55fce2e66b60)
    at ../src/backends/native/meta-seat-impl.c:1999
#5  process_event (event=0x7f8534034cf0, seat_impl=<optimized out>)
    at ../src/backends/native/meta-seat-impl.c:2495
#6  process_events (seat_impl=<optimized out>) at ../src/backends/native/meta-seat-impl.c:2506
#7  0x00007f85a50f855a in dispatch_libinput (seat_impl=0x55fce2e66b60)
    at ../src/backends/native/meta-seat-impl.c:252
#8  meta_event_dispatch
    (g_source=<optimized out>, callback=<optimized out>, user_data=<optimized out>)
    at ../src/backends/native/meta-seat-impl.c:1426
#9  0x00007f85a5c703d7 in g_main_dispatch (context=0x55fce2e5de80) at ../glib/gmain.c:3337
#10 g_main_context_dispatch (context=0x55fce2e5de80) at ../glib/gmain.c:4055
#11 0x00007f85a5cc4518 in g_main_context_iterate.constprop.0
    (context=0x55fce2e5de80, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>)
    at ../glib/gmain.c:4131
#12 0x00007f85a5c6f9a3 in g_main_loop_run (loop=0x7f8534033510) at ../glib/gmain.c:4329
#13 0x00007f85a5101421 in input_thread (seat_impl=0x55fce2e66b60)
--Type <RET> for more, q to quit, c to continue without paging--c
    at ../src/backends/native/meta-seat-impl.c:2683
#14 0x00007f85a5c9ece2 in g_thread_proxy (data=0x55fce2d780c0) at ../glib/gthread.c:826
#15 0x00007f85a441c269 in start_thread (arg=0x7f85623f7640) at pthread_create.c:473
#16 0x00007f85a4ed3653 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

(gdb) p priv->monitor_manager
Cannot access memory at address 0xffffffffffffff20
(gdb) p priv
$1 = (MetaBackendPrivate *) 0xffffffffffffff20
(gdb) l
1133    MetaMonitorManager *
1134    meta_backend_get_monitor_manager (MetaBackend *backend)
1135    {
1136      MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
1137
1138      return priv->monitor_manager;
1139    }
1140
1141    /**
1142     * meta_backend_get_orientation_manager: (skip)


Version-Release number of selected component (if applicable):
gnome-shell-40.0~beta-2.20210304git7a57528bd.fc34.x86_64
mutter-40.0~beta-1.fc34.x86_64
gtk3-3.24.26-1.fc34.x86_64
gtk4-4.1.1-1.fc34.x86_64

How reproducible:
This gnome-shell crash happened 3 of 3 times I've logged out of GNOME 40 beta on Wayland

Steps to Reproduce:
1. Boot a Fedora 34 KDE Plasma installation with the GNOME desktop group installed updated to 2021-3-7 with updates testing enabled
2. Log in to GNOME 40 beta on Wayland
3. Log out

Actual results:
gnome-shell 40 beta segmentation faulted in meta_backend_get_monitor_manager when logging out

Expected results:
No crash would happen.

Additional info:
gnome-shell crashes in meta_backend_get_monitor_manager were reported at 
https://gitlab.gnome.org/GNOME/mutter/-/issues/1650
https://gitlab.gnome.org/GNOME/mutter/-/issues/1586
https://gitlab.gnome.org/GNOME/mutter/-/issues/1676