Refactor: wrong usage of iterator().next()

There are many usages of `iterator().next()` -- but it can raise `NoSuchElementException` error on empty list, so all related code must check it before call `next()`.

Search code for: `iterator().next()`

There are ~150 places to check.

Good code example:

Bad code and fixes example:
640e5394c9d433ea07de57f4b22fee9dadd7dcb5, 0db46ba3dacf34945fa9ba27fbfbe0c9cfd43a37

Search the codebase for every `iterator().next()` usage, then compare each case with the good and bad examples in the issue and the referenced commits. Review whether the collection can be empty and ensure all roughly 150 occurrences handle that case safely; done means the search finds no unreviewed usages.

Written by the indexing model from the issue text.