potentially fix plugins despawning the entity the moment it appears

src/main/java/me/youhavetrouble/standin/entity/EntityHandler.java

@@ -117,6 +117,7 @@ public abstract class EntityHandler<E extends Entity> {
                     if (foundConverter == null) return;
                     Entity converted = foundConverter.spawn(existing);
                     if (converted == null) return;
+                    if (!converted.isInWorld()) return;
                     existing.remove();
                 }, ClickCallback.Options.builder().lifetime(Duration.ofHours(1)).uses(1).build())
         ).build();