Opening a file read-only, that is already opened, breaks

[sedrubal]

When opening a file, that is already opened with another instance of (neo-)vim, you usually see this error message:

E325: ATTENTION
Found a swap file by the name ...

Then you have the choices:

[O]pen Read-Only, (E)dit anyway, (R)ecover, (Q)uit, (A)bort: 

When you choose [O]pen Read-Only or (R)ecover, you see this error message:

Error detected while processing function <SNR>71_startup:
line    7:

...which seems to belong to the exec call in s:startup function:

file-line/plugin/file_line.vim

Lines 76 to 88 in 559088a

	function! s:startup()
	autocmd BufNewFile * nested call s:gotoline()
	autocmd BufRead * nested call s:gotoline()
	if argc() > 0
	let argidx=argidx()
	silent call s:handle_arg()
	exec (argidx+1).'argument'
	" Manually call Syntax autocommands, ignored by `:argdo`.
	doautocmd Syntax
	doautocmd FileType
	endif
	endfunction

I have no glue, what the function does, but (argidx+1).'argument' seems to be 1argument at this point.