Error: Assertion failed: Attempted to removeChild with a node that was not a child.
at window.assertions.assertFunction (http://phettest.colorado.edu/assert/js/assert.js:23:13)
at Node.removeChild (http://phettest.colorado.edu/scenery/js/nodes/Node.js?bust=1467735321428:374:17)
at Array.removalListener (http://phettest.colorado.edu/curve-fitting/js/curve-fitting/view/BucketAndGraphAreaNode.js?bust=1467735321428:76:26)
at ObservableArray._fireItemRemoved (http://phettest.colorado.edu/axon/js/ObservableArray.js?bust=1467735321428:163:18)
at ObservableArray.remove (http://phettest.colorado.edu/axon/js/ObservableArray.js?bust=1467735321428:203:14)
at end (http://phettest.colorado.edu/curve-fitting/js/curve-fitting/view/BucketAndGraphAreaNode.js?bust=1467735321428:97:42)
at SimpleDragHandler.endDrag (http://phettest.colorado.edu/scenery/js/input/SimpleDragHandler.js?bust=1467735321428:165:26)
at Object.up (http://phettest.colorado.edu/scenery/js/input/SimpleDragHandler.js?bust=1467735321428:74:19)
at Input.dispatchToPointer (http://phettest.colorado.edu/scenery/js/input/Input.js?bust=1467735321428:811:29)
at Input.dispatchEvent (http://phettest.colorado.edu/scenery/js/input/Input.js?bust=1467735321428:784:14)