gravity"blocks behaving with cobwebs

putting 1 to 10 cobewebs what so ever and a block above this cobwebs and put a (gravel,sand,anvil,gravity blocks) destroy the block the gravel falls into the cobewebs here is the bug when it falls it falls slowly (thats normal) but as soon as u count to 5 while its falling in the cobewebs the gravel disappear and app crashing !!!!!