CraftComplexPart should return getParent()'s status in isValid

src/main/java/org/bukkit/craftbukkit/entity/CraftComplexPart.java

@@ -27,6 +27,11 @@ public class CraftComplexPart extends CraftEntity implements ComplexEntityPart {
         return getParent().getLastDamageCause();
     }
 
+    @Override
+    public boolean isValid() {
+        return getParent().isValid();
+    }
+
     @Override
     public EntityComplexPart getHandle() {
         return (EntityComplexPart) entity;