1. override
void didUnmountRenderObject(RenderCustomPaint renderObject)

A render object previously associated with this widget has been removed from the tree. The given RenderObject will be of the same type as returned by this object's createRenderObject.

Source 

@override
void didUnmountRenderObject(RenderCustomPaint renderObject) {
  renderObject
    ..painter = null
    ..foregroundPainter = null;
}