Bufix in Pages: Avoid calling a nil function. Fixes #85

pages.go

@@ -217,6 +217,9 @@ func (p *Pages) HasFocus() bool {
 
 // Focus is called by the application when the primitive receives focus.
 func (p *Pages) Focus(delegate func(p Primitive)) {
+	if delegate == nil {
+		return // We cannot delegate so we cannot focus.
+	}
 	p.setFocus = delegate
 	var topItem Primitive
 	for _, page := range p.pages {