Ensure regen step not running on the main thread (#2185)

worldedit-bukkit/src/main/java/com/fastasyncworldedit/bukkit/adapter/Regenerator.java

@@ -253,10 +253,13 @@ public abstract class Regenerator<IChunkAccess, ProtoChunk extends IChunkAccess,
                     e.printStackTrace();
                 }
             } else { // Concurrency.NONE or generateConcurrent == false
-                // run sequential
+                // run sequential but submit to different thread
+                // running regen on the main thread otherwise triggers async-only events on the main thread
+                executor.submit(() -> {
                     for (long xz : coords) {
                         chunkStatus.processChunkSave(xz, worldlimits.get(radius).get(xz));
                     }
+                }).get(); // wait until finished this step
             }
         }