Onboard makes Xserver segfault in _XkbSetMapChecks

The bug has been reported against onboard but seems rather an Xorg issue since the X server is hitting a SIGSEGV https://bugs.launchpad.net/onboard/+bug/1809737

Using current Ubuntu and xserver 1.20.4

• start a GNOME session
• install onboard, start it, hold e.g the 'g' letter
• select on the special chars in the popup list

-> Xorg segfault with that backtrace

#0  0x00005576d497d28d in _XkbSetMapChecks
    (client=client@entry=0x5576d6e6a920, req=req@entry=0x5576d70a0f00, values=values@entry=0x5576d70a0f24 "", dev=<optimized out>)
    at ../../../../xkb/xkb.c:2387
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x68: 
        xkbi = #1  0x00005576d4984707 in ProcXkbSetMap (client=0x5576d6e6a920)
    at ../../../../xkb/xkb.c:2655
        other = 0x5576d6b47b40
        dev = 0x5576d6b9d610
        tmp = 0x5576d70a0f24 ""
        rc = 0
        stuff = <optimized out>
        master = 0x5576d6b9d610
#2  0x00005576d488299e in Dispatch () at ../../../../dix/dispatch.c:478
        result = <optimized out>
        client = 0x5576d6e6a920
        start_tick = 1470
#3  0x00005576d4886986 in dix_main
    (argc=12, argv=0x7ffe80f5b858, envp=<optimized out>)
    at ../../../../dix/main.c:276
        i = <optimized out>
        alwaysCheckForInput = {0, 1}
#4  0x00007f08e7413b6b in __libc_start_main (main=
    0x5576d4870640 <main>, argc=12, argv=0x7ffe80f5b858, init=<optimized out>, f--Type <RET> for more, q to quit, c to continue without paging--
ini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffe80f5b848)
    at ../csu/libc-start.c:308
        result = <optimized out>
        unwind_buf = 
              {cancel_jmp_buf = {{jmp_buf = {0, 3226940716949917152, 93968860120656, 140731062007888, 0, 0, 8779993601240944096, 8661551008833162720}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x7ffe80f5b8c0, 0x7f08e8556190}, data = {prev = 0x0, cleanup = 0x0, canceltype = -2131380032}}}
        not_first_call = <optimized out>
#5  0x00005576d487067a in _start ()

(one similar problem was described on https://github.com/andyholmes/gnome-shell-extension-gsconnect/issues/323 but it doesn't look like it was upstreamed)