Address crash with empty main menu

Fixes fyne-io#1073

internal/driver/glfw/menu.go

@@ -5,7 +5,15 @@ import (
 )
 
 func buildMenuOverlay(menus *fyne.MainMenu, c fyne.Canvas) fyne.CanvasObject {
-	if menus.Items[0].Items[len(menus.Items[0].Items)-1].Label != "Quit" { // make sure the first menu always has a quit option
+	if len(menus.Items) == 0 {
+		fyne.LogError("Main menu must have at least one child menu", nil)
+		return nil
+	}
+	var firstItem *fyne.MenuItem
+	if len(menus.Items[0].Items) > 0 {
+		firstItem = menus.Items[0].Items[len(menus.Items[0].Items)-1]
+	}
+	if firstItem == nil || firstItem.Label != "Quit" { // make sure the first menu always has a quit option
 		quitItem := fyne.NewMenuItem("Quit", func() {
 			fyne.CurrentApp().Quit()
 		})

internal/driver/glfw/menu_test.go

@@ -0,0 +1,33 @@
+package glfw
+
+import (
+	"testing"
+
+	"fyne.io/fyne"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Menu_Empty(t *testing.T) {
+	w := createWindow("Menu Test").(*window)
+	bar := buildMenuOverlay(fyne.NewMainMenu(), w.canvas)
+	assert.Nil(t, bar) // no bar but does not crash
+}
+
+func Test_Menu_AddsQuit(t *testing.T) {
+	w := createWindow("Menu Test").(*window)
+	mainMenu := fyne.NewMainMenu(fyne.NewMenu("File"))
+	bar := buildMenuOverlay(mainMenu, w.canvas)
+	assert.NotNil(t, bar)
+	assert.Equal(t, 1, len(mainMenu.Items))
+	assert.Equal(t, 2, len(mainMenu.Items[0].Items)) // separator+quit inserted
+}
+
+func Test_Menu_LeaveQuit(t *testing.T) {
+	w := createWindow("Menu Test").(*window)
+	mainMenu := fyne.NewMainMenu(fyne.NewMenu("File", fyne.NewMenuItem("Quit", nil)))
+	bar := buildMenuOverlay(mainMenu, w.canvas)
+	assert.NotNil(t, bar)
+	assert.Equal(t, 1, len(mainMenu.Items[0].Items)) // no separator added
+	assert.Nil(t, mainMenu.Items[0].Items[0].Action) // no quit handler added
+}