Ensure your phone has at least 50% battery to prevent it from shutting down during a critical process.
Once you have completed your flashing or troubleshooting, you need to exit Fastboot. Do not just pull the battery (you can't on the P Smart 2019). How to Open Fastboot Mode on HUAWEI P Smart 2019
fastboot devices
Go to Settings > About Phone and tap Build Number seven times until it says "You are now a developer." Ensure your phone has at least 50% battery