Nil dereference in button renderer when Icon field is set directly

[drognisep]

Describe the bug:

When a button's Icon field is set directly instead of through SetIcon, then it effectively cannot be disabled due to a nil dereference of b.button.disabledIcon in the renderer.

To Reproduce:

Steps to reproduce the behaviour:

1. Create a button by setting the Icon field directly.
2. Disable the button by calling theButton.Disable().

Where the crash happens:

func (b *buttonRenderer) Refresh() {
 /* ... */
			if b.button.Disabled() {
				// if the icon has changed, create a new disabled version
				// if we could be sure that button.Icon is only ever set through the button.SetIcon method, we could remove this
				if !strings.HasSuffix(b.button.disabledIcon.Name(), b.button.Icon.Name()) { // <<< boom
					b.icon.Resource = theme.NewDisabledResource(b.button.Icon)
				} else {
					b.icon.Resource = b.button.disabledIcon
				}
			} else {
				b.icon.Resource = b.button.Icon
			}
/* ... */
}

Example code:

func main() {
    myApp := app.New()
    win := myApp.NewWindow("Testing bug")

    var crashButton *widget.Button
    crashButton = &widget.Button{
        Text:       "Crash",
        Icon:       theme.ConfirmIcon(),
        Importance: widget.HighImportance,
        OnTapped: func() {
            crashButton.Disable() // <<< bombs in renderer
        },
    }
    content := fyne.NewContainerWithLayout(layout.NewVBoxLayout(),
        layout.NewSpacer(), crashButton, layout.NewSpacer(),
    )
    time.AfterFunc(1 * time.Second, func() {
        dialog.ShowCustom("Testing bug", "Don't crash", content, win)
    })
    win.ShowAndRun()
}

Device (please complete the following information):

• OS: Windows 10
• Version: 2004 build 19041.630
• Go version: go version go1.15.3 windows/amd64
• Fyne version: 1.4.0

[andydotxyz]

On release/v1.4.x for testing