Null-ls setup function complains when receives an argument

[flppv]

It's weird for me that I can't find such issue from someone else, so I might have missed something myself, this is the only plugin I am getting such warning

What should I do to get rid of it?

[jose-elias-alvarez]

This is just a warning from lua-language-server, which attempts to infer type information, so you can safely ignore it. Having said that, I don't have the same issue on my end:

Are you using lua-dev.nvim? That'll give you proper type information when editing your config files.