Kudzu / space vines stop spreading a few seconds after planted (with runtimes!)

[Angustmeta]

Observed on Terry today, and reproduced locally.

Reproduction:

1. Plant a kudzu seed on ground.

Result: It stops spreading after a few moments. If you destroy the borders it regrows only what you destroyed. Then it starts throwing runtimes every couple of seconds (see runtime 2 below).

This could NOT be reproduced on my build from Jan 12, 2022, where #63812 was merged, so that PR is likely NOT to blame. Kudzu did still throw other runtimes on that build however, after mutating.

This non-spread bug could be reproduced after updating late Jan 13, 2022. Meaning the problem is likely caused by a very recent merge.

Runtimes:

Runtime 1 - Some time after planting

[00:07:51] Runtime in spacevine.dm,569: Cannot execute null.Enter().

proc name: spread (/obj/structure/spacevine/proc/spread)
src: the space vine (/obj/structure/spacevine)
src.loc: null
call stack:
the space vine (/obj/structure/spacevine): spread()
/datum/spacevine_controller (/datum/spacevine_controller): process(2)
Objects (/datum/controller/subsystem/processing/obj): fire(0)
Objects (/datum/controller/subsystem/processing/obj): ignite(0)
Master (/datum/controller/master): RunQueue()
Master (/datum/controller/master): Loop()
Master (/datum/controller/master): StartProcessing(0)

Runtime 2 - Spams after destroying all kudzu except one tile, I think

00:13:13] Runtime in spacevine.dm,517: Cannot read null.mutations

Another error, also seen on the build from January 12, 2022.

TESTING: GC: -- [0x2005bcb] | /obj/structure/spacevine was unable to be GC'd --

[the-orange-cow]

Saw this same thing happen just now on Round # 176551.

[LemonInTheDark]

likely my fault somehow

[ShizCalev]

@LemonInTheDark Yup. The process block is runtiming if vine in growth_queue is being QDEL'd
https://github.com/tgstation/tgstation/pull/63877/files#diff-918db9eb6b54e91a2ea210af60b5c2409f1eda6459a54ce60b814fe74d1c1b79

[LemonInTheDark]

Oh fuck I see