Hi, what I did was literally illogical, but anyway I did it and now I am sorry..
I was playing in my 1.12.2 world, which is very important for me. And I was having fun by summoning mobs. I summoned some enderdragons, but I noticed they stayed for a second, then vanished. I wondered what happened to them, so I typed /tp @e[type=minecraft:ender_dragon], and then I teleported somewhere, but the world crashed, saying "invalid move player packet received". And now every time I load the world I am getting kicked with this message.
I don't know what to do now. This world means a lot to me.
Say something silly, Laugh 'til it hurts, Take a risk, Sing out loud, Rock the boat, Shake things up, Flirt with disaster, Buy something frivolous, Color outside the lines, Cause a scene, Order dessert, Make waves, Get carried away, Have a great day!
You can fix the world by downloading NBTExplorer and editing level.dat (in the root directory of the world save) by opening the "Player" entry and editing the "Pos" tag (a list of 3 numbers) to have to valid numbers (such as 0 100 0, or set them to "Spawn[XYZ]", which is the player's last spawn point if you ever slept in a bed or used /spawnpoint; otherwise, the world spawn point can be used, which is "Spawn[XYZ]" in the root of level.dat).