Placing a survival structure builder for BridgeEmpireMedium29blockspan appears to immediately cause the game to crash, with subsequent attempts to load the effected world results in repeated crashes before the world finishes loading. My latest test produced the attached crash reports.

crash-2019-03-05_19.34.05-server.txt
crash-2019-03-05_19.34.09-client.txt

I have added an additional null check that should prevent this crash, but it likely happens only in combination with some other mod (probably core one) that makes it so that structure builder gets update run before it's setup correctly which doesn't happen if the mod is in pack by itself.