Make sure type parameter of Event class is non-null

[kafumi]

• null can't be used as a valid content of Event because Event uses null to indicate the event has been already handled
• It means the type parameter T of Event should never be nullable
• Expressed that constrain by giving upper bound Any to T
  • Reference: https://speakerdeck.com/npryce/the-kotlin-type-hierarchy-from-top-to-bottom?slide=27