Revisit using null safe modifiers

In #54 we made an attempt of introducing `@Nullable` and `@Notnull` modifiers. Unfortunately reverted the change since it introduced some warnings which need to be addressed somehow. Probably it's OK to go with them.

Start by reviewing the reverted attempt from issue #54 and the warnings shown in the attached screenshots. Determine which nullability modifiers can be restored without warnings; done means the annotations are reintroduced and the reported warnings are addressed.

Written by the indexing model from the issue text.