Possibly, you shouldn't define realm as a class property, but rather locally in a do-try-catch closure where you perform some realm action (say, realm.write). 
Moreover, at the line of the error: if the statementlet realm = try! Realm fails, you will hit an assert which will cause that the app to crash at runtime (thanks @marius).
Instead, consider using your calls to Realm() in some class method, where you handle possible errors/asserts via the catch closure. 
func TryToWriteToRealm(myText: String) {
    do {
        let realm = try Realm()
        try realm.write {
            realm.add(myText)
        }
    } catch {
        print("Error!")
        // some error handling
    }
}
Then call TryToWriteToRealm(...) when you want to e.g. Write to your realm. 
For details of the difference between try, try? and try!, see e.g.