Update WorldEditHandler.java

Author: TylerS1066

src/main/java/io/github/eirikh1996/structureboxes/WorldEditHandler.java

@@ -258,6 +258,7 @@ public void run() {
                 session.flushSession();
                 sbMain.clearInterior(interior);
                 structure.setPlacementTime(System.currentTimeMillis());
+                session.close();
             } catch (WorldEditException e) {
                 e.printStackTrace();
             }