Comment # 8 on bug 1200076 from Stefan Dirsch

#7  _XkbSetMapChecks (client=client@entry=0x55f87ca18080, 
    req=req@entry=0x55f87ca3cd40, values=values@entry=0x55f87ca3cd64 "", 
    dev=<optimized out>) at xkb.c:2487
#8  0x000055f879559adf in ProcXkbSetMap (client=0x55f87ca18080) at xkb.c:2760

xkb.c
[...]
2474 _XkbSetMapChecks(ClientPtr client, DeviceIntPtr dev, xkbSetMapReq * req,
2475                 char *values)
2476 {
[...]
2486    xkbi = dev->key->xkbInfo;
2487    xkb = xkbi->desc;
[...]
2760            rc = _XkbSetMapChecks(client, other, stuff, tmp);
2761            if (rc != Success)
2762                return rc;

So this looks like a Null pointer dereference (xkbi == NULL)