prevent (and alert on) multiple instances of fzf-lua

lua/fzf-lua/core.lua

@@ -10,6 +10,7 @@ local M = {}
 M.fzf = function(opts, contents)
   -- setup the fzf window and preview layout
   local fzf_win = win(opts)
+  if not fzf_win then return end
   -- instantiate the previewer
   -- if not opts.preview and not previewer and
   local previewer, preview_opts = nil, nil

lua/fzf-lua/win.lua

@@ -171,6 +171,10 @@ function FzfWin:reset_win_highlights(win, is_border)
 end
 
 function FzfWin:new(o)
+  if _self then
+    utils.warn("Please close fzf-lua before starting a new instance")
+    return nil
+  end
   o = o or {}
   self = setmetatable({}, { __index = self })
   self.winopts = normalize_winopts(o)