fix: Error on goto_location when code window closed

Mentioned by @gennaro-tedesco in #91

lua/outline/sidebar.lua

@@ -386,6 +386,11 @@ function Sidebar:__goto_location(change_focus)
     return
   end
 
+  if not vim.api.nvim_win_is_valid(self.code.win) then
+    vim.notify("outline.nvim: Code window closed", vim.log.levels.WARN)
+    return
+  end
+
   vim.api.nvim_win_set_cursor(self.code.win, { node.line + 1, node.character })
 
   if cfg.o.outline_window.center_on_jump then