You have a series of debugMsg calls that check for if you're in a dev environment, which is good. Except, in using Boolean.toString(test_shield.hasEvent()) as an expression in the parameters on call, you are still attempting to build a string using the test_shield even in production code. And in production, test_shield is uninitialized so it NPEs, crashing the game on startup. Please consider null checks, or doing the entire test system within one large if (FabricLoader.getInstance().isDevelopmentEnvironment()) block.