World chunks loading error

I use Tlauncher to play Minecraft. I used to play 1.14.4 but my computer was too weak to play so i decided to update to 1.15. Everything was better then. But yesterday, it told me to update Minecraft 1.15. I thought that was normal. That kind of notification always appears in 1.14.4. Then I joined my world and saw these errors. I was shocked and worried. That is my favorite world, so I can't delete it. I don't need Mojang to fix my world but they have to fix this now. Maybe someone met these errors too?