getFileType contains this piece of code:

if get(g:, 'loaded_nvim_treesitter', 0)
    let type = luaeval('require"emmet_utils".get_node_at_cursor()')
" […]

However the assumptions here are flawed. g:loaded_nvim_treesitter is v:true, even if there is no treesitter parser or grammar for the current filetype. Thus type gets set to v:null in files with only regex syntax highlighting. This then causes an error later down the line when trying to call len(type). There needs to be an alternative or more thorough check, that treesitter actually returns a file type.

The error is: E701: Invalid type for len()