The null reference exception occurs when the Bricklayers mod is installed.
Fails on line 95 of the RockManager class because block.Code is null and fails when trying to match.
foreach (Block block in api.World.Blocks)
{
if(block.Code != null) {
if (WildcardUtil.Match(rockCode, block.Code))
Comments on api.World.Blocks:
List of all loaded blocks. The array index is the block id. Some may be null or placeholders (then block.code is null). Client-side none are null, what was null return as air blocks.