If you are starting from scratch and designing a brand new language then ADT's are a clearly better choice than allowing null. However Dart is not a brand new l

If you are starting from scratch and designing a brand new language then ADT's are a clearly better choice than allowing null. However Dart is not a brand new language. It's a preexisting language that already has null. Given that context nullable types are the only option that doesn't break compatibility with previous languages and allows code to be made null safe. If you already have to have null values because of prior decisions then ADT's don't add null safety. There may be other reasons to want them that aren't about handling null safety. But you'll still have the problem that values can be null and you won't have given developers a tool to handle those potentially null values safely.