Fix Forge regen, by retaining the world reference. Also close the world.

worldedit-forge/src/main/java/com/sk89q/worldedit/forge/ForgeWorld.java

@@ -96,6 +96,7 @@ import net.minecraft.world.storage.WorldInfo;
 
 import javax.annotation.Nullable;
 import java.io.File;
+import java.io.IOException;
 import java.lang.ref.WeakReference;
 import java.nio.file.Path;
 import java.util.Collections;
@@ -329,8 +330,8 @@ public class ForgeWorld extends AbstractWorld {
 
             MinecraftServer server = originalWorld.getServer();
             SaveHandler saveHandler = new SaveHandler(saveFolder, originalWorld.getSaveHandler().getWorldDirectory().getName(), server, server.getDataFixer());
-            World freshWorld = new ServerWorld(server, server.getBackgroundExecutor(), saveHandler, originalWorld.getWorldInfo(),
-                    originalWorld.dimension.getType(), originalWorld.getProfiler(), new NoOpChunkStatusListener());
+            try (World freshWorld = new ServerWorld(server, server.getBackgroundExecutor(), saveHandler, originalWorld.getWorldInfo(),
+                    originalWorld.dimension.getType(), originalWorld.getProfiler(), new NoOpChunkStatusListener())) {
 
                 // Pre-gen all the chunks
                 // We need to also pull one more chunk in every direction
@@ -343,6 +344,9 @@ public class ForgeWorld extends AbstractWorld {
                 for (BlockVector3 vec : region) {
                     editSession.setBlock(vec, from.getFullBlock(vec));
                 }
+            } catch (IOException e) {
+                throw new RuntimeException(e);
+            }
         } catch (MaxChangedBlocksException e) {
             throw new RuntimeException(e);
         } finally {