Programmers use Null as different flags. It can hint an absence, an undefined value, en error etc.
Multiple semantics lead to coupling and errors.

TL;DR: Null is schizofrenic and does not exist in real world. His creator regreted and programmers around the world suffer it. Don't be a part of it.

Problems

• Coupling among the callers and the senders.

• Mismatch among the callers and the senders.

• If/Switch/Case Polluting.

• Null is not polymorphic with real objects. Hence, Null Pointer Exception

• Null does not exist on real world. Thus, it violates Bijection Principle

Solutions

• Avoid Null. 

• Use NullObject pattern to avoid ifs.

• Use Optionals.