ClassNotFoundException when calling a custom event via mixin

[Vekhove]

With my own custom event:

    public static class ExampleEvent implements IEvent<Void> {
        private final String string;

        public ExampleEvent(String string) {
            this.string = string;
        }

        public String getString() {
            return this.string;
        }
    }

and the mixin:

@Mixin(Player.class)
public final class PlayerMixin {

    @Inject(
            method = "notifyPickupItem",
            at = @At("HEAD")
    )
    public void mixintests_injectNotifyPickupItem(Ref<EntityStore> ref, ItemStack itemStack, Vector3d position, ComponentAccessor<EntityStore> componentAccessor, CallbackInfo ci) {
        System.out.println("Picked up!");

        IEventDispatcher<EventTests.ExampleEvent, EventTests.ExampleEvent> dispatcher = HytaleServer.get().getEventBus().dispatchFor(EventTests.ExampleEvent.class);
        dispatcher.dispatch(new EventTests.ExampleEvent("From Mixin Pickup!"));
    }

}

causes the error:

org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:381)
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:237)
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
	at com.build_9.hyxin.HyxinTransformer.transform(HyxinTransformer.java:81)
	at com.hypixel.hytale.plugin.early.TransformingClassLoader.transformAndDefine(TransformingClassLoader.java:97)
	at com.hypixel.hytale.plugin.early.TransformingClassLoader.loadClass(TransformingClassLoader.java:80)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:490)
	at com.hypixel.hytale.server.core.modules.entity.EntityModule.setup(EntityModule.java:343)
	at com.hypixel.hytale.server.core.plugin.PluginBase.setup0(PluginBase.java:389)
	at com.hypixel.hytale.server.core.plugin.PluginManager.setup(PluginManager.java:757)
	at com.hypixel.hytale.server.core.plugin.PluginManager.setup(PluginManager.java:236)
	at com.hypixel.hytale.server.core.HytaleServer.boot(HytaleServer.java:326)
	at com.hypixel.hytale.server.core.HytaleServer.<init>(HytaleServer.java:284)
	at com.hypixel.hytale.LateMain.lateMain(LateMain.java:54)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
	at java.base/java.lang.reflect.Method.invoke(Method.java:565)
	at com.hypixel.hytale.Main.launchWithTransformingClassLoader(Main.java:64)
	at com.hypixel.hytale.Main.main(Main.java:40)
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinPreProcessorException: Attach error for mixintests.mixins.json:PlayerMixin from mod (unknown) during activity: [Transform -> Method mixintests_injectNotifyPickupItem(Lcom/hypixel/hytale/component/Ref;Lcom/hypixel/hytale/server/core/inventory/ItemStack;Lcom/hypixel/hytale/math/vector/Vector3d;Lcom/hypixel/hytale/component/ComponentAccessor;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfo;)V -> INVOKESPECIAL -> me/lilac/eventtests/EventTests$ExampleEvent::<init>:(Ljava/lang/String;)V]
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:313)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.createContextFor(MixinPreProcessorStandard.java:277)
	at org.spongepowered.asm.mixin.transformer.MixinInfo.createContextFor(MixinInfo.java:1292)
	at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:203)
	at org.spongepowered.asm.mixin.transformer.TargetClassContext.apply(TargetClassContext.java:437)
	at org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:418)
	at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:352)
	... 17 more
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: me.lilac.eventtests.EventTests$ExampleEvent
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transformMemberReference(MixinPreProcessorStandard.java:791)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transformMethod(MixinPreProcessorStandard.java:777)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transform(MixinPreProcessorStandard.java:743)
	at org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:307)
	... 23 more
Caused by: java.lang.ClassNotFoundException: me.lilac.eventtests.EventTests$ExampleEvent
	... 27 more

Calling a vanilla event the same way does not cause an error.
Calling the event the same way elsewhere, like in a command, works as expected.

This may be something I am doing wrong or something not possible, but any help or info would be great!

[cerus]

This happens for all classes in the plugin, not just events. I assume it's some sort of class loader problem?

[Diamantino-Op]

Is the mixin a separate Jar? If yes, make sure it loads after your main plugin.

[Vekhove]

Is the mixin a separate Jar? If yes, make sure it loads after your main plugin.

Hyxin is an earlyplugin and loads the mixins before the server even starts, my plugin is loaded when regular plugins/mods load. How can I make it load after my plugin?