here is my solution
distanceWidget.set('map', null);
and distanceWidget is:
distanceWidget = new DistanceWidget({
map: map,
distance: rads,
maxDistance: 0.5,
color: '#000000',
activeColor: '#5599bb',
sizerIcon: '../Contents/Images/resize-off.png',
activeSizerIcon: '../Contents/Images/resize.png'
});