It seems weston is not properly checking checking that shsurf->output is not NULL before trying to use it to get the width here.

Backtrace:

#0  set_fullscreen (shsurf=0x1ce5ed70, fullscreen=fullscreen@entry=true, output=output@entry=0x0) at ../desktop-shell/shell.c:2700
#1  0x0000007f95dba7e0 in desktop_surface_fullscreen_requested (desktop_surface=<optimized out>, fullscreen=true, output=0x0, shell=<optimized out>) at ../desktop-shell/shell.c:2796
#2  0x0000007f95d94d00 in weston_desktop_api_fullscreen_requested (desktop=<optimized out>, surface=surface@entry=0x1ce5e9d0, fullscreen=fullscreen@entry=true, output=output@entry=0x0)
    at ../libweston-desktop/libweston-desktop.c:224
#3  0x0000007f95d98694 in weston_desktop_xdg_toplevel_protocol_set_fullscreen (wl_client=<optimized out>, resource=<optimized out>, output_resource=<optimized out>) at ../libweston-desktop/xdg-shell.c:539
#4  0x0000007f97730468 in ffi_call_SYSV () at ../src/aarch64/sysv.S:114
#5  0x0000007f9772ff28 in ffi_call_int (cif=cif@entry=0x7fd3d3eca0, fn=0x1ce6b1b0, orig_rvalue=0x7fd3d3eb38, orig_rvalue@entry=0x0, avalue=avalue@entry=0x7fd3d3eb40, closure=0x1ce0b000, closure@entry=0x0)
    at ../src/aarch64/ffi.c:747
#6  0x0000007f9773010c in ffi_call (cif=cif@entry=0x7fd3d3eca0, fn=<optimized out>, rvalue=rvalue@entry=0x0, avalue=avalue@entry=0x7fd3d3eb40) at ../src/aarch64/ffi.c:756
#7  0x0000007f978dd970 in wl_closure_invoke (closure=closure@entry=0x1cd71260, flags=flags@entry=2, target=<optimized out>, target@entry=0x1ce6b1b0, opcode=opcode@entry=11, data=<optimized out>, 
    data@entry=0x1ce0b000) at src/connection.c:1014
#8  0x0000007f978daa70 in wl_client_connection_data (fd=<optimized out>, mask=<optimized out>, data=0x1ce0b000) at src/wayland-server.c:429
#9  0x0000007f978db974 in wl_event_source_fd_dispatch (source=<optimized out>, ep=<optimized out>) at src/event-loop.c:95
#10 0x0000007f978dc0f4 in wl_event_loop_dispatch (loop=0x1cd58250, timeout=timeout@entry=-1) at src/event-loop.c:641
#11 0x0000007f978dac68 in wl_display_run (display=display@entry=0x1cd58400) at src/wayland-server.c:1293
#12 0x0000007f97adfbbc in wet_main (argc=<optimized out>, argv=0x7fd3d3f4a8) at ../compositor/main.c:3360
#13 0x00000000004006ac in main (argc=<optimized out>, argv=<optimized out>) at ../compositor/executable.c:33