What is “Fatal error: Unexpectedly found nil while unwrapping an Optional value”?

A swift variable can be any of the following types:

var anVar: Int = 10
var anOptionalVar: Int? = 42 
var anNonOptionVar: Int!  //Have no valid value, this value is implicitly unwrapped, usually used in dependecy injection
var anUninitializedOptionalVar: Int? // `nil` is the default

An optional variable is a type that represents either a wrapped value or nil, the absence of a value.

To access a value for an option variable, you have to unwrap it. There are five ways to unwrap an optional.

Nil Cascading operator:

This is one of the safest ways to access Option. This usually never crashes:

var anOptionalVar: Int? = 42
if let nonOptionalVar = anOptionalVar {
    print("\(nonOptionalVar)")
} else {
    print("nonOptionalVar is nil")
}

Optional Chaining:

To safely access the properties and methods of a wrapped instance, use the postfix optional chaining operator (postfix ?). This usually never crashes:

class Student {
   let name: String?
   let address: String? 
}
var student: Student?
if student?.name == nil {
    print("not a valid name")
}

Forced unwrapping: