Fix bugs with appTabs render (#3983)

Fixes #3980

container/apptabs.go

@@ -277,18 +277,22 @@ type appTabsRenderer struct {
 
 func (r *appTabsRenderer) Layout(size fyne.Size) {
 	// Try render as many tabs as will fit, others will appear in the overflow
-	for i := len(r.appTabs.Items); i > 0; i-- {
-		r.updateTabs(i)
-		barMin := r.bar.MinSize()
-		if r.appTabs.location == TabLocationLeading || r.appTabs.location == TabLocationTrailing {
-			if barMin.Height <= size.Height {
-				// Tab bar is short enough to fit
-				break
-			}
-		} else {
-			if barMin.Width <= size.Width {
-				// Tab bar is thin enough to fit
-				break
+	if len(r.appTabs.Items) == 0 {
+		r.updateTabs(0)
+	} else {
+		for i := len(r.appTabs.Items); i > 0; i-- {
+			r.updateTabs(i)
+			barMin := r.bar.MinSize()
+			if r.appTabs.location == TabLocationLeading || r.appTabs.location == TabLocationTrailing {
+				if barMin.Height <= size.Height {
+					// Tab bar is short enough to fit
+					break
+				}
+			} else {
+				if barMin.Width <= size.Width {
+					// Tab bar is thin enough to fit
+					break
+				}
 			}
 		}
 	}

container/apptabs_internal_test.go

@@ -47,3 +47,20 @@ func Test_tabButtonRenderer_DeleteAdd(t *testing.T) {
 	tabs.SelectTab(item2)
 	assert.Equal(t, pos, indicator.Position())
 }
+
+func Test_tabButtonRenderer_EmptyDeleteAdd(t *testing.T) {
+	item1 := &TabItem{Text: "Test", Content: widget.NewLabel("Content")}
+	tabs := NewAppTabs()
+
+	// ensure enough space for buttons to be created.
+	tabs.Resize(fyne.NewSize(300, 200))
+
+	tabRenderer := cache.Renderer(tabs).(*appTabsRenderer)
+	assert.Equal(t, 0, len(tabRenderer.bar.Objects[0].(*fyne.Container).Objects))
+
+	tabs.Append(item1)
+	assert.Equal(t, 1, len(tabRenderer.bar.Objects[0].(*fyne.Container).Objects))
+
+	tabs.Remove(item1)
+	assert.Equal(t, 0, len(tabRenderer.bar.Objects[0].(*fyne.Container).Objects))
+}