Is it a normal practice to consider every parameter/variable coming from android framework nullable? I feel It's not ergonomic to deal with optionality everywhere even though some variables can never be null. Is there any cheeatsheet or anything I should read on to know which parameters can be null or not?

What do you mean “every parameter/variable”? Not all Android Framework API are nulalble, many of them are nullable, some non-nullable, but still most of them has platform type (Kotlin doesn’t know about nullability)

The source code of the recent Android SDK is now marked up as Nullable / NonNull, but Android Support Library is not yet. We have to be careful when overriding onActivityResult, onSaveInstanceState, etc., on AppCompatActivity.

Support Library has nullable annotations too, especially for version 27. And only very small part of Android Framework API is marked with nullability annotations (I may be wrong, but I don't see many annotations on AF)

I use Support Library Revision 27.0.2 , at least it's AppCompatActivity is not markuped.

You can check sources of AppCompatActivity, there is a bunch of Nullable and NonNull annotations

oh, you're right. The functions I mentioned were overridden from Framework's FragmentActivity. It was not a Nullable markup.