I've been following a tutorial on youtube for flutter.

CodePudding user response：

In the video you linked, there is a check whether points[x] is null, so I assume the elements of points should be nullable. You can achieve this with

List<Offset?> points = [];

(instead of List<Offset> points = [];). The added questionmark makes it possible for elements of the list to be either a instance of the class Offset or the value null.

CodePudding user response：

The points is non-nullable list.

So this time, it can be solved with to remove points.add(null); inside onPanEnd. (Or add non-null Offset.)