ios - How to get sorted Dictionary from MutableArray using swift3 -

i manage create dictionary database add sections in tableview.

my function database array , array dictionary (a: aname1,aname2, b: bname1,bname2...). when sorted dictionary return value type [string, [string]] , need [string: [string]], when don't use sort action it's give error:

thread1:exc_bas_instruction(code=exc_i386_invop,subcode=0x0)

in console:

fatal error: unexpectedly found nil while unwrapping optional value (lldb)

my code:

class sectiondata {      var marrcrimesnames : nsmutablearray!      func getcrimesnames() {         marrcrimesnames = nsmutablearray()         marrcrimesnames = modelmanager.getinstance().getallcrimesnames()     }      func getsectionsfromdata() -> [string: [string]] {          var sectiondictionary =  [string: [string]]()          let crime: crimesinfo = (marrcrimesnames.object(at: 0) as! crimesinfo) // thread1:exc_bas_instruction(code=exc_i386_invop,subcode=0x0)         _ = crime.name         var firstletter: [string] = []          crime in marrcrimesnames {             firstletter.append((crime anyobject).name)         }          let characters = array(set(firstletter.flatmap({ $0.characters.first })))          character in characters.map({ string($0) }) {             sectiondictionary[character] = firstletter.filter({ $0.hasprefix(character) })         }          //var sortedsectiondictionary = sectiondictionary.sorted(by: { $0.key < $1.key })         // sort action return [string, [string]]          return sectiondictionary     }  } 

the error assume value return in: let crime: crimesinfo = (marrcrimesnames.object(at: 0) as! crimesinfo) set.

the error show not set , return nil. check it:

if let crime = marrcrimesnames.object(at: 0) {    // here can safely use crime ;) }