java.lang.NoClassDefFoundError: net/minecraft/client/audio/ISound
at lach_01298.qmd.item.QMDItems.init(QMDItems.java:120) ~[QMDItems.class:?]
at lach_01298.qmd.proxy.CommonProxy.preInit(CommonProxy.java:37) ~[CommonProxy.class:?]
at lach_01298.qmd.QMD.preInit(QMD.java:42) ~[QMD.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_275]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_275]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_275]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_275]
at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:632) ~[mohist-1.12.2-185-server.jar:?]
at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source) ~[?:?]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_275]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_275]
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$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) ~[minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.EventBus.post(EventBus.java:217) ~[minecraft_server.1.12.2.jar:?]
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:215) ~[mohist-1.12.2-185-server.jar:?]
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:193) ~[mohist-1.12.2-185-server.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_275]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_275]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_275]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_275]
at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) ~[minecraft_server.1.12.2.jar:?]
at com.google.common.eventbus.Subscriber$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:132) ~[LoadController.class:?]
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:644) ~[Loader.class:?]
at net.minecraftforge.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:102) ~[FMLServerHandler.class:?]
at net.minecraftforge.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:329) ~[FMLCommonHandler.class:?]
at net.minecraft.server.dedicated.DedicatedServer.init(DedicatedServer.java:168) ~[nz.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:596) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_275]
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.audio.ISound
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(ClassLoader.java:418) ~[?:1.8.0_275]
at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_275]
... 38 more
Caused by: java.lang.NullPointerException

It seems like related to referencing client-only classes while initializing the gluon gun

fixed in 0.12.2