Back button killed webview

[diyism]

package main

import
(
	"image/color"
	"log"
	"os"

	"gioui.org/app"
	"gioui.org/io/system"
	"gioui.org/layout"
	"gioui.org/op"
	"gioui.org/text"
	"gioui.org/widget/material"
	"gioui.org/font/gofont"

	"github.com/inkeliz/gowebview"
)

func main()
{	go func()
	{	w := app.NewWindow()
		if err := loop(w); err != nil
		{	log.Fatal(err)
		}
		os.Exit(0)
	}()
	app.Main()
}

func loop(w *app.Window) error
{	th := material.NewTheme(gofont.Collection())
	var ops op.Ops
	var wv  gowebview.WebView
	var has_back bool
	for
	{	e:=<-w.Events()
		switch e:=e.(type)
		{	case app.ViewEvent:
				if wv==nil
				{	go func()
					{	var err error
						wv,err = gowebview.New(&gowebview.Config{URL:"https://google.com/ncr", WindowConfig:&gowebview.WindowConfig{Title:"Hello World", Window:e.View, VM:app.JavaVM()}})
						if err!=nil {panic(err)}
						defer wv.Destroy()
						wv.Run()
					}()
				} else
				{	wv.SetVisibility(gowebview.VisibilityMaximized)
					log.Println("===================resume webview==============")
				}
			case *system.CommandEvent:
				if e.Type==system.CommandBack
				{	if has_back==false
					{	log.Println("===================back button==============", wv)
						e.Cancel = true
						wv.SetVisibility(gowebview.VisibilityMinimized)
						has_back=true
					} else
					{	log.Println("===================back button second time==============", wv)
						e.Cancel=false
						has_back=false
					}
				}
			case system.DestroyEvent:
				return e.Err
			case system.FrameEvent:
				gtx := layout.NewContext(&ops, e)
				l := material.H1(th, "Hello, gowebview")
				maroon := color.NRGBA{R: 127, G: 0, B: 0, A: 255}
				l.Color = maroon
				l.Alignment = text.Middle
				l.Layout(gtx)
				e.Frame(gtx.Ops)
		}
	}
}

javac -source 8 -target 8 -bootclasspath $ANDROID_SDK_ROOT/platforms/android-29/android.jar -d /tmp/gowebview/classes /opt/lib/go/src/github.com/inkeliz/gowebview/gowebview_android.java

jar cf /opt/lib/go/src/github.com/inkeliz/gowebview/gowebview_android.jar -C /tmp/gowebview/classes .