Do not prevent entity gc if a local listener is added

src/main/java/net/minestom/server/event/EventNodeLazyImpl.java

@@ -5,6 +5,7 @@ import org.jetbrains.annotations.NotNull;
 
 import java.lang.invoke.MethodHandles;
 import java.lang.invoke.VarHandle;
+import java.lang.ref.WeakReference;
 import java.util.function.Consumer;
 
 final class EventNodeLazyImpl<E extends Event> extends EventNodeImpl<E> {
@@ -18,13 +19,13 @@ final class EventNodeLazyImpl<E extends Event> extends EventNodeImpl<E> {
         }
     }
 
-    private final Object owner;
+    private final WeakReference<Object> owner;
     @SuppressWarnings("unused")
     private boolean mapped;
 
     EventNodeLazyImpl(@NotNull Object owner, @NotNull EventFilter<E, ?> filter) {
         super(owner.toString(), filter, null);
-        this.owner = owner;
+        this.owner = new WeakReference<>(owner);
     }
 
     @Override
@@ -59,6 +60,10 @@ final class EventNodeLazyImpl<E extends Event> extends EventNodeImpl<E> {
 
     private void ensureMap() {
         if (MAPPED.compareAndSet(this, false, true)) {
+            final Object owner = this.owner.get();
+            if (owner == null) {
+                throw new IllegalStateException("Node handle is null. Be sure to never cache a local node.");
+            }
             MinecraftServer.getGlobalEventHandler().map(this, owner);
         }
     }

src/test/java/net/minestom/server/entity/EntityRemovalIntegrationTest.java

@@ -3,6 +3,7 @@ package net.minestom.server.entity;
 import net.minestom.server.api.Env;
 import net.minestom.server.api.EnvTest;
 import net.minestom.server.coordinate.Pos;
+import net.minestom.server.event.entity.EntityTickEvent;
 import net.minestom.server.network.packet.server.play.DestroyEntitiesPacket;
 import net.minestom.server.utils.time.TimeUnit;
 import org.junit.jupiter.api.Test;
@@ -78,6 +79,23 @@ public class EntityRemovalIntegrationTest {
         waitUntilCleared(ref);
     }
 
+    @Test
+    public void entityNodeGC(Env env) {
+        // Ensure that the entities GCed when a local listener is present
+        var node = env.process().eventHandler();
+        var entity = new Entity(EntityType.ZOMBIE);
+        entity.eventNode().addListener(EntityTickEvent.class, event -> {
+        });
+        node.call(new EntityTickEvent(entity));
+
+        var ref = new WeakReference<>(entity);
+        entity.remove();
+        //noinspection UnusedAssignment
+        entity = null;
+        env.tick();
+        waitUntilCleared(ref);
+    }
+
     static final class TestEntity extends Entity {
         public TestEntity(long delay, TemporalUnit unit) {
             super(EntityType.ZOMBIE);