fix(#2632): occasional error stack when locating nvim-tree window (#2633)

fix: passing nil as window handle in view.get_winnr

Co-authored-by: Alexander Courtis <alex@courtis.org>

Author: usachev63

lua/nvim-tree/view.lua

@@ -464,7 +464,7 @@ end
 function M.get_winnr(tabpage)
   tabpage = tabpage or vim.api.nvim_get_current_tabpage()
   local tabinfo = M.View.tabpages[tabpage]
-  if tabinfo ~= nil and vim.api.nvim_win_is_valid(tabinfo.winnr) then
+  if tabinfo and tabinfo.winnr and vim.api.nvim_win_is_valid(tabinfo.winnr) then
     return tabinfo.winnr
   end
 end