The bug
When an item frame is somewhere in the loaded parts of the world, the following error message appears in the server console:
Exception loading entity:
java.lang.NullPointerException: null
at ati.f(SourceFile:86) ~[server-1.14.3-pre3.jar:?]
at ath.c(SourceFile:232) ~[server-1.14.3-pre3.jar:?]
at ail.<init>(SourceFile:234) ~[server-1.14.3-pre3.jar:?]
at ath.<init>(SourceFile:35) ~[server-1.14.3-pre3.jar:?]
at ati.<init>(SourceFile:46) ~[server-1.14.3-pre3.jar:?]
at aip.a(SourceFile:422) ~[server-1.14.3-pre3.jar:?]
at aip.b(SourceFile:431) ~[server-1.14.3-pre3.jar:?]
at java.util.Optional.map(Unknown Source) ~[?:1.8.0_201]
at aip.a(SourceFile:431) ~[server-1.14.3-pre3.jar:?]
at aip.b(SourceFile:484) ~[server-1.14.3-pre3.jar:?]
at aip.a(SourceFile:465) ~[server-1.14.3-pre3.jar:?]
at bya.a(SourceFile:393) ~[server-1.14.3-pre3.jar:?]
at bya.b(SourceFile:151) ~[server-1.14.3-pre3.jar:?]
at bxo.w(SourceFile:484) ~[server-1.14.3-pre3.jar:?]
at uu.a(SourceFile:594) ~[server-1.14.3-pre3.jar:?]
at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[server-1.14.3-pre3.jar:?]
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[server-1.14.3-pre3.jar:?]
at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[server-1.14.3-pre3.jar:?]
at uu.a(SourceFile:583) ~[server-1.14.3-pre3.jar:?]
at java.util.concurrent.CompletableFuture.uniApply(Unknown Source) ~[?:1.8.0_201]
at java.util.concurrent.CompletableFuture$UniApply.tryFire(Unknown Source) ~[?:1.8.0_201]
at java.util.concurrent.CompletableFuture$Completion.run(Unknown Source) ~[?:1.8.0_201]
at uw.b(SourceFile:57) ~[server-1.14.3-pre3.jar:?]
at agq.e(SourceFile:83) [server-1.14.3-pre3.jar:?]
at agq.a(SourceFile:123) [server-1.14.3-pre3.jar:?]
at agq.run(SourceFile:91) [server-1.14.3-pre3.jar:?]
at agn.h(SourceFile:135) [server-1.14.3-pre3.jar:?]
at agn.p(SourceFile:114) [server-1.14.3-pre3.jar:?]
at vg$a.p(SourceFile:506) [server-1.14.3-pre3.jar:?]
at vg.d(SourceFile:234) [server-1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.aW(SourceFile:707) [server-1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.p(SourceFile:695) [server-1.14.3-pre3.jar:?]
at agn.c(SourceFile:123) [server-1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.o(SourceFile:680) [server-1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:469) [server-1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:372) [server-1.14.3-pre3.jar:?]
at ui.d(SourceFile:218) [server-1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:604) [server-1.14.3-pre3.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_201]
As a result, the item frame that was about to be loaded will disappear.
When the client tries to place an item frame, the following error message will appear in the server console:
Error executing task on Server
java.lang.NullPointerException: null
at ati.f(SourceFile:86) ~[1.14.3-pre3.jar:?]
at ath.c(SourceFile:232) ~[1.14.3-pre3.jar:?]
at ail.<init>(SourceFile:234) ~[1.14.3-pre3.jar:?]
at ath.<init>(SourceFile:35) ~[1.14.3-pre3.jar:?]
at ath.<init>(SourceFile:39) ~[1.14.3-pre3.jar:?]
at ati.<init>(SourceFile:50) ~[1.14.3-pre3.jar:?]
at bbw.a(SourceFile:40) ~[1.14.3-pre3.jar:?]
at bce.a(SourceFile:189) ~[1.14.3-pre3.jar:?]
at vk.a(SourceFile:312) ~[1.14.3-pre3.jar:?]
at wa.a(SourceFile:951) ~[1.14.3-pre3.jar:?]
at pi.a(SourceFile:36) ~[1.14.3-pre3.jar:?]
at pi.a(SourceFile:10) ~[1.14.3-pre3.jar:?]
at kd.a(SourceFile:15) ~[1.14.3-pre3.jar:?]
at kd$$Lambda$3080/1958423162.run(Unknown Source) ~[?:?]
at ri.run(SourceFile:18) ~[1.14.3-pre3.jar:?]
at agn.h(SourceFile:135) [1.14.3-pre3.jar:?]
at agr.h(SourceFile:23) [1.14.3-pre3.jar:?]
at agn.p(SourceFile:114) [1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.aW(SourceFile:701) [1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.p(SourceFile:695) [1.14.3-pre3.jar:?]
at agn.c(SourceFile:123) [1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.o(SourceFile:680) [1.14.3-pre3.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:635) [1.14.3-pre3.jar:?]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51]
Additionally, the client will crash with the following crash report.
Crash report
1.14.3 Pre-Release 3\: [^crash-2019-06-14_11.35.28-client.txt]
Description: Unexpected error
java.lang.NullPointerException: Unexpected error
at ati.f(SourceFile:86)
at ath.c(SourceFile:232)
at ail.<init>(SourceFile:234)
at ath.<init>(SourceFile:35)
at ath.<init>(SourceFile:39)
at ati.<init>(SourceFile:50)
at bbw.a(SourceFile:40)
at bce.a(SourceFile:189)
at dhq.a(SourceFile:321)
at cvo.aF(SourceFile:1335)
at cvo.aG(SourceFile:1562)
at cvo.m(SourceFile:1411)
at cvo.e(SourceFile:925)
at cvo.b(SourceFile:411)
at net.minecraft.client.main.Main.main(SourceFile:154)
Original description
When updating the server to the latest 1.14.3 pre-release 3, at first start up the server console spams a lot of errors. After restart these errors are gone, but when logging on to the server, all item frames are gone.
Reproduce: put in latest world backup with item frames, then start server again. This will give multiple errors. When restarting there are no errors and all item frames are gone.
Also tested without datapacks, but still same issue
Client side: when placing item frame, the client crashes. This causes error on the server, but server remains running.
Client crash report included in attachments.
Attachments
Comments

Blocks MC-132199
I don't think it's worth bothering with all the issues that are potentially blocked by this one. Every issue that needs item frames to be reproduced is blocked by this. I don't expect this bug to last long.
I can confirm this, my item frames disapeared from chests and trying to place them again cause crash to desktop.
I also can confirm this... Even in a new world the placing of item frames will crash the game
Thank you MOD for linking my post. I DID do a search, but I didn't find this.
To confirm: All frames are gone. Placing new ones crashes game.
Going back to pre2 :-/
EDIT: Just for curiosities sake, I opened my 1.14.3pre3 world in 1.14.3pre2 (to see if it somehow had saved the item frame data) and the item frames, the items within them, etc etc, are all gone. Sooo... Hopefully you didn't do much in this pre-release without a backup
I experienced this today and noticed it as soon as I spawned in. definitely not caused by data packs because I had removed mine to start my save file in 1.14.3 pre release3
Minecraft 1.14.3 pre-release 3, vanilla version
Linux Mint 19
The same happens to clocks.
Also, can confirm everything said here.
@J.J. Berkhout If I had to venture a guess, I would be willing to bet it's any entity that is attached to a block. They said in the patch notes that they changed the way torches (and the like) attach to glass panes, iron bars, (etc).

@Rellik PanDragon guessing is nice, but in this case not right. So far itemframes and clocks. I tried more stuff that hangs to the wall, torches (and the like), but they are good. So, for now I just have seen it with itemframes and clocks.
Same here, game crashes when placing an item frame. 😛
Im wondering how such a huge bug got missed before 1.14.3 (pre release 3) got released? Im not entoerly sure how bug fix pre releases work because I usually ignore them do to unstability, but theres a major game breaking bug not fixed in 1.14.2 that I cant play with so suddenly Ive been here since pre release 1 learning all about how this bug report site works and stuff.
Pre-releases are a thing to ensure that no severe bugs exist in the game. You should never open a world that you care about in a snapshot or a pre-release.
As for, "The same happens to clocks"... how exactly does the same happen to clocks? You can't place clocks.
Violine1101: You are right. The way I use the clock is to put it in an item frame on the wall. As I did that quite some time ago, I forgot it WAS in an item frame, so it is just this same bug. Sorry.
any eta on 1.14.3pre4? Normally I wouldn't even bother asking, but man they been cranking these things out like crazy lately, that this long a pause (on such a MAJOR issue) seems crazy lol
I think 1.14.3pre4 will come out sometime this week. If I am reading the description here correctly it has been delayed and should have released 2 days ago. It's possible they delayed either because this bug hasn't been fixed yet, or they are planning on releasing 1.14.3 and wanted to fix some more stuff.
This also happened when item frames were first added. Not crashing, but if item frames were on a wall, they would disappear if you walked away from them.

Please keep comments on topic, every comment (and every edit of a comment) triggers 33 mails. Since this report is resolved, pretty much the only good reason to comment is to claim that it was not fixed. Over 6 year old issues are extremely unlikely to be related.
Can confirm. This is not caused by any data packs. Seems like the server throws an error message and the client crashes when an item frame is somewhere in the loaded parts of the world