I am getting this error today:

Error while compiling: RaidableBases.cs(9483,75): error CS1503: Argument `#3' cannot convert `null' expression to type `bool'

Does anyone know what to do to fix this?

upgrade to the latest version

ya, you need to update as MrLiquid stated. that error is specific to the WaterLevel changes in the Rust update which was fixed in 2.7.3