[MC-187583] Closing game during Mojang loading screen causes errors

The bug

When closing the game while the Mojang load screen is shown, errors occur.
Depending on when you close the game, the logged errors will differ:

1.16-pre1 log (Closing at beginning of load screen)

[11:54:11] [main/INFO]: Stopping!
[11:54:11] [Worker-Main-8/WARN]: Exception loading blockstate definition: minecraft:blockstates/mossy_stone_brick_wall.json: java.io.FileNotFoundException: minecraft:blockstates/mossy_stone_brick_wall.json
[11:54:11] [Worker-Main-8/WARN]: Exception loading blockstate definition: 'minecraft:blockstates/mossy_stone_brick_wall.json' missing model for variant: 'minecraft:mossy_stone_brick_wall#east=tall,north=tall,south=low,up=false,waterlogged=true,west=none'
[11:54:11] [Worker-Main-8/WARN]: Exception loading blockstate definition: 'minecraft:blockstates/mossy_stone_brick_wall.json' missing model for variant: 'minecraft:mossy_stone_brick_wall#east=none,north=low,south=none,up=false,waterlogged=false,west=tall'
[11:54:11] [Worker-Main-8/WARN]: Exception loading blockstate definition: 'minecraft:blockstates/mossy_stone_brick_wall.json' missing model for variant: 'minecraft:mossy_stone_brick_wall#east=none,north=none,south=low,up=false,waterlogged=false,west=none'
...
[11:54:11] [Worker-Main-6/ERROR]: Couldn't load texture minecraft:textures/font/unicode_page_00.png
java.io.FileNotFoundException: minecraft:textures/font/unicode_page_00.png
	at abh.a(SourceFile:67) ~[1.16-pre1.jar:?]
	at dpb.a(SourceFile:107) [1.16-pre1.jar:?]
	at dpb$$Lambda$2771/1010334754.apply(Unknown Source) [1.16-pre1.jar:?]
	at java.util.HashMap.computeIfAbsent(HashMap.java:1118) [?:1.8.0_51]
	at dpb.a(SourceFile:85) [1.16-pre1.jar:?]
	at doo$1.a(SourceFile:115) [1.16-pre1.jar:?]
	at doo$1$$Lambda$2719/448831122.accept(Unknown Source) [1.16-pre1.jar:?]
	at it.unimi.dsi.fastutil.ints.IntIterable.forEach(IntIterable.java:74) [fastutil-8.2.1.jar:?]
	at doo$1.a(SourceFile:110) [1.16-pre1.jar:?]
	at doo$1.b(SourceFile:51) [1.16-pre1.jar:?]
	at abf.a(SourceFile:11) [1.16-pre1.jar:?]
	at abf$$Lambda$2605/1295675429.get(Unknown Source) [1.16-pre1.jar:?]
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582) [?:1.8.0_51]
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1574) [?:1.8.0_51]
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) [?:1.8.0_51]
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) [?:1.8.0_51]
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689) [?:1.8.0_51]
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) [?:1.8.0_51]
...

1.16-pre1 log (Closing near end of load screen)

[11:48:46] [main/INFO]: Reloading ResourceManager: Default
[11:48:52] [main/INFO]: Stopping!
[11:48:52] [Worker-Main-10/ERROR]: Using missing texture, unable to load minecraft:textures/block/respawn_anchor_bottom.png : java.io.FileNotFoundException: minecraft:textures/block/respawn_anchor_bottom.png
[11:48:52] [Worker-Main-10/ERROR]: Using missing texture, unable to load minecraft:textures/block/anvil.png : java.io.FileNotFoundException: minecraft:textures/block/anvil.png
[11:48:52] [Worker-Main-10/ERROR]: Using missing texture, unable to load minecraft:textures/block/composter_top.png : java.io.FileNotFoundException: minecraft:textures/block/composter_top.png
[11:48:52] [Worker-Main-10/ERROR]: Using missing texture, unable to load minecraft:textures/block/cartography_table_side1.png : java.io.FileNotFoundException: minecraft:textures/block/cartography_table_side1.png
[11:48:52] [Worker-Main-10/ERROR]: Using missing texture, unable to load minecraft:textures/item/flint.png : java.io.FileNotFoundException: minecraft:textures/item/flint.png
...

1.17.1 log

Shutdown failure!
java.util.ConcurrentModificationException
	at java.base/java.util.HashMap.forEach(HashMap.java:1428)
	at exn.close(SourceFile:173)
	at dvp.close(SourceFile:1038)
	at dvp.l(SourceFile:1017)
	at net.minecraft.client.main.Main.main(SourceFile:233)

24w10a log

[19:05:46] [Render thread/ERROR]: Shutdown failure!
java.util.ConcurrentModificationException: null
	at java.base/java.util.HashMap.forEach(HashMap.java:1424) ~[?:?]
	at glt.close(SourceFile:173) ~[24w10a.jar:?]
	at fby.close(SourceFile:1247) [24w10a.jar:?]
	at fby.n(SourceFile:1221) [24w10a.jar:?]
	at net.minecraft.client.main.Main.main(SourceFile:281) [24w10a.jar:?]

How to reproduce

Start the game
While the Mojang load screen is shown, press the X of the game window
Look at the log