Code (Text):

1. on explosion:
2.     loop exploded blocks:
3.         set block at location of loop-block to stone

None blocks was set, and I get this error:

Code (Text):

1. [15:17:30] [Server thread/ERROR]: Block at 133, 65, -179 is Block{minecraft:stone} but has [email protected]. Bukkit will attempt to fix this, but there may be additional damage that we cannot recover.

Works if put a delay:

Code (Text):

1.     loop exploded blocks:
2.         wait 3 seconds
3.         set block at location of loop-block to stone