Help! My friends minecraft server (hosted locally on my friends computer) keeps crashing.

[Arasander]

So me and my friend decided to start a public SMP with mods (some fabric mods and origin mods which are all fabric) and we tried making the server locally with his laptop because We are broke. So it was all going well until this happened. when he tries to start the server this appears and says stopped server.

java.lang.NullPointerException: Cannot invoke "net.minecraft.class_3218.method_8621()" because "$$5" is null at net.minecraft.server.MinecraftServer.method_3723(MinecraftServer.java:566) ~[server-intermediary.jar:?] at net.minecraft.server.MinecraftServer.method_3782(MinecraftServer.java:613) ~[server-intermediary.jar:?] at net.minecraft.class_3176.method_3782(class_3176.java:567) ~[server-intermediary.jar:?] at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:723) ~[server-intermediary.jar:?] at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:270) ~[server-intermediary.jar:?] at java.lang.Thread.run(Thread.java:833) [?:?]

Please someone help we have been trying to fix this for about a week.