padding property Null safety
Returns the insets to apply when using this decoration on a box that has contents, so that the contents do not overlap the edges of the decoration. For example, if the decoration draws a frame around its edge, the padding would return the distance by which to inset the children so as to not overlap the frame.
This only works for decorations that have absolute sizes. If the padding needed would change based on the size at which the decoration is drawn, then this will return incorrect padding values.
For example, when a BoxDecoration has BoxShape.circle, the padding does not take into account that the circle is drawn in the center of the box regardless of the ratio of the box; it does not provide the extra padding that is implied by changing the ratio.
The value returned by this getter must be resolved (using EdgeInsetsGeometry.resolve to obtain an absolute EdgeInsets. (For example, BorderDirectional will return an EdgeInsetsDirectional for its padding.)
EdgeInsetsGeometry? get padding => EdgeInsets.zero;