How to configure Neovim to use sumneko_lua for all Lua LSP needs?

I configured the sumneko_lua LSP server from kickstart.nvim & in addition used the selene + stylua builtins of the null-ls plugin. But the issue right now is whenever I attempt to write a buffer, Neovim prompts me to choose either sumneko_lua or null-ls as the LSP.

Screenshot attached for reference:

So, how do I configure Neovim to cooperate with null-ls & sumneko_lua for my LSP needs?

NOTE: I’m using Neovim v0.7.0 (not the nightly version)

This is because null-ls and sumneko_lua both support formatting of lua files.

You will find possible solutions here :

Thanks for sharing those resources, I did take a look at them & tried to fix the issue. I’m a Lua noob but here’s what I did & it still doesn’t work for whatever reasons:

local custom_on_attach = function(client, bufnr)
    -- A bunch of keymaps for LSP

local lua_formatting_callback = function(client, bufnr)
    local params = vim.lsp.util.make_formatting_params({})
    client.request("textDocument/formatting", params, nil, bufnr)

    on_attach = function(client, bufnr)
        lua_formatting_callback(client, bufnr)
        custom_on_attach(client, bufnr)
    -- For autocompletion using nvim-cmp
    capabilities = capabilities
    settings = {
        -- Some Lua LSP settings

-- Configuration for null-ls
    sources = {