While the tests that run on autopkgtest infra suggest it still is an issue, I first I tried to reproduce as-is to be sure the trigger is good (new release, new kernel, new OVS):

$ autopkgtest --apt-upgrade --shell --no-built-binaries openvswitch_2.9.0-0ubuntu1.dsc -- qemu --qemu-command=qemu-system-i386 --cpus 4 --ram-size=4096 ~/autopkgtest-bionic-i386.img
$ autopkgtest --apt-upgrade --shell --no-built-binaries openvswitch_2.9.0-0ubuntu1.dsc -- qemu --cpus 4 --ram-size=4096 ~/autopkgtest-bionic-i386.img

While faster in the crash with more CPUs I eventually reduced to 1 to have better (more clear) stack traces.

It hangs (tests POV) and crashes (main console running dmesg -w).

[   56.320025] BUG: unable to handle kernel NULL pointer dereference at 00000000
[   56.320760] IP: add_grec+0x28/0x450
[   56.321137] *pdpt = 000000001ebe7001 *pde = 0000000000000000 
[   56.321699] Oops: 0000 [#1] SMP
[   56.322009] Modules linked in: veth openvswitch nsh nf_conntrack_ipv6 nf_nat_ipv6 nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_defrag_ipv6 nf_nat nf_conntrack libcrc32c 9p fscache kvm_intel ppdev kvm irqbypass joydev 9pnet_virtio input_leds parport_pc serio_raw 9pnet parport qemu_fw_cfg mac_hid sch_fq_codel ip_tables x_tables autofs4 btrfs xor zstd_compress raid6_pq psmouse virtio_blk virtio_net i2c_piix4 pata_acpi floppy
[   56.325571] CPU: 0 PID: 240 Comm: systemd-journal Tainted: G        W        4.15.0-34-generic #37-Ubuntu
[   56.326485] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[   56.327356] EIP: add_grec+0x28/0x450
[   56.327712] EFLAGS: 00010202 CPU: 0
[   56.328052] EAX: 00000000 EBX: dda65420 ECX: 00000006 EDX: dda65420
[   56.328651] ESI: dc489a00 EDI: dc489a00 EBP: d94c9f34 ESP: d94c9ef4
[   56.329259]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
[   56.329774] CR0: 80050033 CR2: 00000000 CR3: 1e9adba0 CR4: 000006f0
[   56.330379] Call Trace:
[   56.330623]  <SOFTIRQ>
[   56.330864]  mld_ifc_timer_expire+0x10e/0x260
[   56.331285]  ? igmp6_timer_handler+0x60/0x60
[   56.331699]  call_timer_fn+0x2f/0x120
[   56.332066]  ? igmp6_timer_handler+0x60/0x60
[   56.332489]  run_timer_softirq+0x3b5/0x410
[   56.332899]  ? rcu_process_callbacks+0xc8/0x470
[   56.333353]  ? __softirqentry_text_start+0x8/0x8
[   56.333808]  __do_softirq+0xae/0x255
[   56.334163]  ? __softirqentry_text_start+0x8/0x8
[   56.334617]  call_on_stack+0x45/0x50
[   56.334971]  </SOFTIRQ>
[   56.335219]  ? irq_exit+0xb5/0xc0
[   56.335549]  ? smp_apic_timer_interrupt+0x6c/0x120
[   56.336022]  ? apic_timer_interrupt+0x3c/0x44
[   56.336451] Code: 74 26 00 3e 8d 74 26 00 55 89 e5 57 56 53 89 c6 83 ec 34 89 4d e8 65 a1 14 00 00 00 89 45 f0 31 c0 f6 42 44 08 8b 42 10 89 45 cc <8b> 00 c7 45 ec 00 00 00 00 0f 85 f1 01 00 00 8b 80 54 01 00 00
[   56.338295] EIP: add_grec+0x28/0x450 SS:ESP: 0068:d94c9ef4
[   56.338832] CR2: 0000000000000000
[   56.339163] ---[ end trace 6b06ace1457ab251 ]---
[   56.339616] Kernel panic - not syncing: Fatal exception in interrupt
[   56.340448] Kernel Offset: 0x9000000 from 0xc1000000 (relocation range: 0xc0000000-0xdf7fdfff)
[   56.341293] ---[ end Kernel panic - not syncing: Fatal exception in interrupt


With that, try the new kernel.

Note: console after starting the test
$ sudo nc -U /tmp/autopkgtest-qemu*/ttyS0

Umm, I was stopped in my tracks realizing this is an amd64 kernel.
@Jsalisbury - I'll need i386 kernels to do this.