NullReferenceException (NRE) is a special kind of exception that is raised when trying to access a member of a
nil reference, such as calling a method or reading/writing the property of an uninitialized field or variable.
In Swift, NREs can happen when force-unwrapping a variable that contains
nil with the
! operator, or when accessing a variable that is declared to implicitly unwrap (by the
! type name suffix) that has not been initialized to a non-
nil value yet.
In the other languages, NREs can happen when accessing any not-non-nullable variable that may be
null, or when force-passing a
null value to a non-nullable parameter or variable.