Nil pointer when getting context's flagSet in UsageError function.

[WUST-mengqinyu]

my urfave/cli version is

v2.3.0

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
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Dependency Management

• My project is using go modules.
• My project is using vendoring.
• My project is automatically downloading the latest version.
• I am unsure of what my dependency management setup is.

Describe the bug

When I try to get flag from UsageError function, program panic because cli.Context's flagSet is a nil pointer.

To reproduce

package main

import (
	"fmt"

	"github.com/urfave/cli/v2"
)

func main() {
	app := cli.NewApp()
	app.Commands = []*cli.Command{
		{
			Name: "test",
			Flags: []cli.Flag{
				&cli.StringFlag{
					Name: "flag",
				},
			},
			OnUsageError: UsageError,
		},
	}
	err := app.Run([]string{"@app", "test", "--flag", "1", "--notflag", "1"})
	fmt.Println(err)
}

func UsageError(ctx *cli.Context, err error, isSubcommand bool) error {
	flag := ctx.String("flag")
	fmt.Println(flag)
	return nil
}

Observed behavior

panic: runtime error: invalid memory address or nil pointer dereference

[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x114b8aa]

Expected behavior

I expect cli.Context can Lineage correctly and find flag I need.

Additional context

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Run go version and paste its output here

go version go1.16 darwin/amd64

Run go env and paste its output here

# paste `go env` output in here