I don't think this is correct. For some tweens (e.g. ColorTween, https://master-api.flutter.dev/flutter/animation/ColorTween-class.html) null has a special meaning. And as documented in https://master-api.flutter.dev/flutter/animation/Tween-class.html under "Nullability" that's totally ok.

I understand that.

1. I haven't removed the ability for the value to be null. I'm just making sure that when the tween should not animate, begin and end should be the same, as there is no animation. This adds to a state of being unanimated.
2. In the current implementation of ImplicitlyAnimatedWidget, it is not possible to animate from non-null color to null anyway, as when the color (aka targetValue) becomes null, the tween is set to null, ignoring it. As a matter of fact, this is something that needs to be fixed too.
3. Lastly, ImplicitlyAnimatedWidget should be agnostic of what should happen when targetValue becomes null. Current extensions of ImplicitlyAnimatedWidget are trying to expect different things from it. Example: AnimatedContainer has a nullable alignment argument which is supposed to be ignored when it is null, which makes its tween null. Whereas AnimatedColor (fictional widget working over ColorTween) has a special meaning when its color is null, meaning its tween should remain non-null and have end set to null.

Possible solution:

1. The tweens' nullability should be handled by ImplicitlyAnimatedWidget's extension instead of itself.
2. The check for whether a tween should animate or not, should depend only on its current end, even it it's null.

   bool _shouldAnimateTween(Tween<dynamic> tween, dynamic targetValue) {
       return targetValue != tween.end;
     }

3. From the perspective of ImplicitlyAnimatedWidget, targetValue being null should be considered as one of the valid tween states instead of using it to null the tween.

From the user's perspective, whatever value (null or non-null) is passed to forEachTween, the tween will animate to it. If I wish to ignore it, I should make the tween null themselves.