BassebombeCraft.java:45: error: cannot find symbol
import net.minecraftforge.fml.common.SidedProxy;

The usage of SidedProxy is deprecated with 1.13:
https://mcforge.readthedocs.io/en/1.13.x/concepts/sides/ which states:
Luckily, FML has DistExecutor, which provides various methods to run different methods on different physical sides, or a single method only on one side.

The solution uses implementation of IProxy as seen here:
https://github.com/McJty/YouTubeModding14/blob/master/src/main/java/com/mcjty/mytutorial/setup/ServerProxy.java

Resolved with commit e3cd519.