I have a forge server for me and my friends but every time I open up the window it closes can someone please help

---- Minecraft Crash Report ----
// Sorry

Time: 12/31/21 11:20 AM
Description: Exception ticking world

java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.remove(HashMap.java:1487)
    at on.d(SourceFile:198)
    at oo.d(SourceFile:206)
    at net.minecraft.server.MinecraftServer.D(SourceFile:624)
    at nz.D(SourceFile:349)
    at net.minecraft.server.MinecraftServer.C(SourceFile:560)
    at net.minecraft.server.MinecraftServer.run(SourceFile:464)
    at java.lang.Thread.run(Thread.java:748)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Server thread
Stacktrace:
    at java.util.HashMap$HashIterator.remove(HashMap.java:1487)
    at on.d(SourceFile:198)
    at oo.d(SourceFile:206)

-- Affected level --
Details:
    Level name: world
    All players: 0 total; []
    Chunk stats: ServerChunkCache: 624 Drop: 369
    Level seed: -8638068294106506447
    Level generator: ID 00 - default, ver 1. Features enabled: true
    Level generator options: 
    Level spawn location: World: (44,64,256), Chunk: (at 12,4,0 in 2,16; contains blocks 32,0,256 to 47,255,271), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
    Level time: 249984 game time, 249984 day time
    Level dimension: 0
    Level storage version: 0x04ABD - Anvil
    Level weather: Rain time: 23977 (now: false), thunder time: 41635 (now: false)
    Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
    at net.minecraft.server.MinecraftServer.D(SourceFile:624)
    at nz.D(SourceFile:349)
    at net.minecraft.server.MinecraftServer.C(SourceFile:560)
    at net.minecraft.server.MinecraftServer.run(SourceFile:464)
    at java.lang.Thread.run(Thread.java:748)

-- System Details --
Details:
    Minecraft Version: 1.12.2
    Operating System: Mac OS X (x86_64) version 10.14.6
    Java Version: 1.8.0_311, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 451528920 bytes (430 MB) / 552599552 bytes (527 MB) up to 1908932608 bytes (1820 MB)
    JVM Flags: 0 total; 
    IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
    Profiler Position: N/A (disabled)
    Player Count: 0 / 20; []
    Is Modded: Unknown (can't tell)
    Type: Dedicated Server (map_server.txt)