fyne-ioGH-2306: use lazy initialization of button

Since the ToolbarAction can also be created
without the constructor, the pointer to the
button could be nil.

widget/toolbar.go

@@ -22,8 +22,16 @@ type ToolbarAction struct {
 
 // ToolbarObject gets a button to render this ToolbarAction
 func (t *ToolbarAction) ToolbarObject() fyne.CanvasObject {
-	t.button.Icon = t.Icon
-	t.button.OnTapped = t.OnActivated
+	if t.button == nil {
+		// lazy initialization of the button, when the ToolbarAction 
+		// has not been created via the constructor
+		t.button = createToolbarActionButton(t.Icon, t.OnActivated)
+	} else {
+		// synchronize properties
+		t.button.Icon = t.Icon
+		t.button.OnTapped = t.OnActivated
+	}
+
 	return t.button
 }
 
@@ -56,12 +64,16 @@ func (t *ToolbarAction) Disabled() bool {
 	return t.button.Disabled()
 }
 
-// NewToolbarAction returns a new push button style ToolbarItem
-func NewToolbarAction(icon fyne.Resource, onActivated func()) *ToolbarAction {
+func createToolbarActionButton(icon fyne.Resource, onActivated func()) *Button {
 	button := NewButtonWithIcon("", icon, onActivated)
 	button.Importance = LowImportance
 
-	return &ToolbarAction{Icon:icon, OnActivated: onActivated, button: button}
+	return button
+}
+
+// NewToolbarAction returns a new push button style ToolbarItem
+func NewToolbarAction(icon fyne.Resource, onActivated func()) *ToolbarAction {
+	return &ToolbarAction{Icon: icon, OnActivated: onActivated, button: createToolbarActionButton(icon, onActivated)}
 }
 
 // ToolbarSpacer is a blank, stretchable space for a toolbar.

widget/toolbar_test.go

@@ -112,7 +112,7 @@ func TestToolbarAction_UpdateOnActivated(t *testing.T) {
 	activated := false
 
 	testIcon := theme.InfoIcon()
-	toolbarAction := NewToolbarAction(testIcon, func() {activated = true})
+	toolbarAction := NewToolbarAction(testIcon, func() { activated = true })
 
 	test.Tap(toolbarAction.ToolbarObject().(*Button))
 
@@ -126,3 +126,10 @@ func TestToolbarAction_UpdateOnActivated(t *testing.T) {
 
 	assert.False(t, activated)
 }
+
+func TestToolbarAction_DefaultCreation(t *testing.T) {
+	testIcon := theme.InfoIcon()
+	toolbarAction := ToolbarAction{Icon: testIcon}
+	obj := toolbarAction.ToolbarObject()
+	assert.Equal(t, testIcon, obj.(*Button).Icon)
+}