so anytime is start a server it says :
[16:54:16] [Server thread/ERROR] [FML]: An error occurred trying to load an EventBusSubscriber com.jozufozu.exnihiloomnia.common.items.ExNihiloItems for modid exnihiloomnia
java.lang.NoClassDefFoundError: net/minecraft/client/util/ITooltipFlag
at java.lang.Class.getDeclaredConstructors0(Native Method) ~[?:1.8.0_292]
at java.lang.Class.privateGetDeclaredConstructors(Class.java:2671) ~[?:1.8.0_292]
at java.lang.Class.getConstructor0(Class.java:3075) ~[?:1.8.0_292]
at java.lang.Class.getConstructor(Class.java:1825) ~[?:1.8.0_292]
at net.minecraftforge.fml.common.eventhandler.EventBus.register(EventBus.java:128) ~[EventBus.class:?]
at net.minecraftforge.fml.common.eventhandler.EventBus.register(EventBus.java:112) ~[EventBus.class:?]
at net.minecraftforge.fml.common.AutomaticEventSubscriber.inject(AutomaticEventSubscriber.java:82) [AutomaticEventSubscriber.class:?]
at net.minecraftforge.fml.common.FMLModContainer.constructMod(FMLModContainer.java:612) [FMLModContainer.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_292]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_292]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_292]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_292]
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76) [minecraft_server.1.12.2.jar:?]
at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.EventBus.post(EventBus.java:217) [minecraft_server.1.12.2.jar:?]
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:219) [LoadController.class:?]
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:197) [LoadController.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_292]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_292]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_292]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_292]
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76) [minecraft_server.1.12.2.jar:?]
at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) [minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.EventBus.post(EventBus.java:217) [minecraft_server.1.12.2.jar:?]
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:136) [LoadController.class:?]
at net.minecraftforge.fml.common.Loader.loadMods(Loader.java:595) [Loader.class:?]
at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:98) [FMLServerHandler.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:333) [FMLCommonHandler.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:125) [nz.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:486) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_292]
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.util.ITooltipFlag
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:418) ~[?:1.8.0_292]
at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_292]
... 39 more
Caused by: java.lang.NullPointerException

i hope thats enugh