Steps to reproduce the issue:

create sub-interface <id> (for a eth interface)
configure ipv6 address on it -> neighbor timer starts
delete sub-interface <name> -> timer kicks in and access a deleted sw_if_index and aborts

Below is the stack trace:
–
(gdb) bt
#0 0x00007ffff65d5c37 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1 0x00007ffff65d9028 in __GI_abort () at abort.c:89
#2 0x00007ffff2d55607 in os_panic () at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vnet/vnet/vnetmain.c:260
#3 0x00007ffff280e8d1 in debugger () at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vppinfra/vppinfra/error.c:84
#4 0x00007ffff280ecd8 in _clib_error (how_to_die=2, function_name=0x0, line_number=0, fmt=0x7ffff32a20d0 "%s:%d (%s) assertion `%s' fails")
at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vppinfra/vppinfra/error.c:143
#5 0x00007ffff2f3f1b5 in vnet_get_sw_interface (vnm=0x7ffff379c3c0 <vnet_main>, sw_if_index=4) at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vnet/vnet/interface_funcs.h:52
#6 0x00007ffff2f3f32a in vnet_sw_interface_get_flags (vnm=0x7ffff379c3c0 <vnet_main>, sw_if_index=4) at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vnet/vnet/interface_funcs.h:113
#7 0x00007ffff2f3f37e in vnet_sw_interface_is_admin_up (vnm=0x7ffff379c3c0 <vnet_main>, sw_if_index=4)
at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vnet/vnet/interface_funcs.h:120
#8 0x00007ffff2f48730 in ip6_neighbor_process_timer_event (vm=0x7ffff379c580 <vlib_global_main>, node=0x7fffb23ad000, frame=0x0)
at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vnet/vnet/ip/ip6_neighbor.c:2011
#9 0x00007ffff2f48cfc in ip6_icmp_neighbor_discovery_event_process (vm=0x7ffff379c580 <vlib_global_main>, node=0x7fffb23ad000, frame=0x0)
at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vnet/vnet/ip/ip6_neighbor.c:2142
#10 0x00007ffff25656ab in vlib_process_bootstrap (_a=140736178379984) at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vlib/vlib/main.c:1191
#11 0x00007ffff2832b3c in clib_calljmp () at /home/ubuntu/development/libfwdd/src/platform/vpp/vpp/build-data/../vppinfra/vppinfra/longjmp.S:110
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