See logged attached.
https://pastebin.com/2XECXdu3
Possible fix (quote by Cam)
https://github.com/TeamOpenIndustry/ImmersiveRailroading/blob/master/src/main/java/cam72cam/immersiverailroading/tile/TileMultiblock.java#L279
if (this.getMultiblock() != null && this.getMultiblock().getInvSize(offset) == 0) {

Confirmed, same issue after trying that line change.

Missed the && vs ||

https://flatworld.crafting.builders/crash-2020-01-13_18.34.50-server.txt