Set HelpFlag=nil causes panic

[wxiaoguang]

My urfave/cli version is

2.25.7

Checklist

• Are you running the latest v2 release? The list of releases is here.
• Did you check the manual for your release? The v2 manual is here
• Did you perform a search about this problem? Here's the GitHub guide about searching.

Describe the bug

The comment says:

// HelpFlag prints the help for all commands and subcommands.
// Set to nil to disable the flag.  The subcommand
// will still be added unless HideHelp or HideHelpCommand is set to true.
var HelpFlag Flag = &BoolFlag{

But the code:

func checkHelp(cCtx *Context) bool {
	found := false
	for _, name := range HelpFlag.Names() {  // !!!!!!!!!  PANIC here
		if cCtx.Bool(name) {

To reproduce

package main

import (
	"github.com/urfave/cli/v2"
	"os"
)

func main() {
	cli.HelpFlag = nil
	app := cli.NewApp()
	_ = app.Run(os.Args)
}

Then panic:

/go/pkg/mod/github.com/urfave/cli/v2@v2.25.7/help.go:419