Don't crash when user hovers over nothing

containerbox.go

@@ -119,10 +119,14 @@ func (this *containerBox) Insert (child, before tomo.Object) {
 
 	beforeBox := assertAnyBox(before.GetBox())
 	index     := indexOf(this.children, tomo.Box(beforeBox))
-	if index < 0 { return }
-
+	
+	if index < 0 {
+		this.children = append(this.children, tomo.Box(box))
+	} else {
+		this.children = insert(this.children, index, tomo.Box(box))
+	}
 	box.setParent(this)
-	this.children = insert(this.children, index, tomo.Box(box))
+
 	this.invalidateLayout()
 	this.invalidateMinimum()
 }

event.go

@@ -339,11 +339,13 @@ func (window *window) handleMotionNotify (
 	}
 
 	underneath := window.boxUnder(image.Pt(x, y), eventCategoryMouse)
-	window.hover(underneath)
-	
-	if !handled {
-		underneath.handleMouseMove()
+	if underneath != nil {
+		window.hover(underneath)
+		if !handled {
+			underneath.handleMouseMove()
+		}
 	}
+	
 }
 
 func (window *window) compressExpose (