Jongseo Park

It seems that you used a translator... it is hard to understand it because of that, but it is unclear if you're saying that you checked the "remember user" box or asking if you need to. But, it seems that you might have unchecked it while trying to check it because it defaults to being checked when you first open the login window, and you might have clicked on it while trying to check the checkbox, which actually unchecked it. I hope that made sense.

I had this issue with forge, but fiddling around with mods folder solved it, I am stating this because you did not state which version of Minecraft you're using, and I need to consider all possibilities, including that you are trying to run forge with mods.

I had a similar issue in the past when a particular mod that is famous for its instability caused a crash, it would make it so that the launcher and windows thought it was still running, and I could not even edit files in the directory. It could be solved by hard-resetting your computer, as when you thought you shut the computer down, Windows actually goes to sleep (even if you tell it to "shut down") and keeps Minecraft in its RAM, so try holding down your power button, unplugging and replugging it (if it is a desktop), or entering this command: "shutdown /s /t 000" into the command prompt or 'run' window that appears when you press Windows+R

WE NEED MORE DETAIL, ok? How did you create that firework? Did you use commands? Did you use a crafting table? If you used a command, a wrong argument in the command may cause it to appear as if it is "small ball green," but render no explosion when shot. 

Did you try removing any mod? Create mod is known to be very complex and requires high-performance. If you remove the mod and the issue is still present, please comment below to let everybody know.

Uh, do you really need or want to do that? I think it would be a good idea to prevent that as empty strings for world names may cause issues in unexpected places.

To achieve that would require extensive modification, either in the way blocks work, or in the way corals are treated, if they change how blocks work, that would be catastrophic as that would mean that Mojang would likely need to re-program how blocks work entirely, if they change how corals work, by either making them as block entity like signs or chests, or making them a full-on entity like armor stands, it would cause extreme lag, as corals would spawn in large quantities, and a full-on entity like an armor stand do not stack, so it would make it impossible to stack corals on top of each other, and they would now be affected by gravity, and it would probably be a nightmare to program them to "die" when they are outside of water (and if they do become a full-on entity, it would be impossible to use /fill command or use worldedit on corals).

It seems that this is a universal issue in 20w20a, me and my cousin each have a Minecraft Java edition snapshot server, and it worked until 20w19a, but it does not work once we update our server and client to 20w20a

A quick search on the internet suggests checking your Firewall.
It also seems that it could be caused by an outdated version of Java.

Please delete this issue and create a new one with same content on the issue tracker for Minecraft Bedrock Edition

Oh, I forgot to remove that!

Thanks!