Fix shared instance unregistering

Signed-off-by: TheMode <themode@outlook.fr>

src/main/java/net/minestom/server/instance/InstanceManager.java

@@ -114,12 +114,12 @@ public final class InstanceManager {
             // Unload all chunks
             if (instance instanceof InstanceContainer) {
                 instance.getChunks().forEach(instance::unloadChunk);
+                var dispatcher = MinecraftServer.process().dispatcher();
+                instance.getChunks().forEach(dispatcher::deletePartition);
             }
             // Unregister
             instance.setRegistered(false);
             this.instances.remove(instance);
-            var dispatcher = MinecraftServer.process().dispatcher();
-            instance.getChunks().forEach(dispatcher::deletePartition);
         }
     }
 

src/test/java/net/minestom/server/api/FlexibleListener.java

@@ -11,6 +11,12 @@ public interface FlexibleListener<E extends Event> {
      */
     void followup(@NotNull Consumer<E> handler);
 
+    default void followup() {
+        followup(event -> {
+            // Empty
+        });
+    }
+
     /**
      * Fails if an event is received. Valid until the next followup call.
      */

src/test/java/net/minestom/server/instance/InstanceUnregisterIntegrationTest.java

@@ -0,0 +1,33 @@
+package net.minestom.server.instance;
+
+import net.minestom.server.api.Env;
+import net.minestom.server.api.EnvTest;
+import net.minestom.server.coordinate.Pos;
+import net.minestom.server.event.player.PlayerTickEvent;
+import org.junit.jupiter.api.Test;
+
+@EnvTest
+public class InstanceUnregisterIntegrationTest {
+
+    @Test
+    public void sharedInstance(Env env) {
+        // Ensure that unregistering a shared instance does not unload the container chunks
+        var instanceManager = env.process().instance();
+        var instance = instanceManager.createInstanceContainer();
+        var shared1 = instanceManager.createSharedInstance(instance);
+        var connection = env.createConnection();
+        var player = connection.connect(shared1, new Pos(0, 40, 0)).join();
+
+        var listener = env.listen(PlayerTickEvent.class);
+        listener.followup();
+        env.tick();
+
+        player.setInstance(instanceManager.createSharedInstance(instance)).join();
+        listener.followup();
+        env.tick();
+
+        instanceManager.unregisterInstance(shared1);
+        listener.followup();
+        env.tick();
+    }
+}