Fix several segfaults

internal/system/box.go

@@ -215,7 +215,7 @@ func (this *box) SetFocusable (focusable bool) {
 }
 
 func (this *box) Focused () bool {
-	hierarchy := this.parent.getHierarchy()
+	hierarchy := this.getHierarchy()
 	if hierarchy == nil { return false }
 	return hierarchy.isFocused(this)
 }

internal/system/hierarchy.go

@@ -52,6 +52,9 @@ func (this *System) NewHierarchy (link WindowLink) *Hierarchy {
 	hierarchy := &Hierarchy {
 		system:      this,
 		link:        link,
+		needMinimum: make(util.Set[anyBox]),
+		needLayout:  make(util.Set[anyBox]),
+		needDraw:    make(util.Set[anyBox]),
 	}
 	return hierarchy
 }

x/window.go

@@ -164,8 +164,12 @@ func (this *Backend) newWindow (
 }
 
 func (this *window) SetRoot (root tomo.Object) {
+	if root == nil {
+		this.hierarchy.SetRoot(nil)
+	} else {
 		this.hierarchy.SetRoot(root.GetBox())
 	}
+}
 
 func (this *window) SetTitle (title string) {
 	this.title = title