Adding a return value in boundary condition.

lib/scatter.js

@@ -410,7 +410,9 @@ export class AbstractScatter extends Throwable {
 
     keepOnStage(velocity, collision = 0.5) {
         let stagePolygon = this.containerPolygon
-        if (!stagePolygon) return
+        // UO: since keepOnStage is called in nextVelocity we need to 
+        // ensure a return value
+        if (!stagePolygon) return { x: 0, y: 0}
         let polygon = this.polygon
         let bounced = this.bouncing()
         if (bounced) {