Swift - completion handler not waiting - swift

Question

My problem is straighforward. I am fetching some data from site with TRON and I need to wait until data is successfully fetched than continue with execution. But my problem is that main thread is not waiting until completion block is completed.
I am doing this as Unit Test, so in setUp() function
Meteorites.Service.sharedInstance.fetchData(completion: { (meteorites, err) in
if err == nil {
self.meteorites = meteorites
}
})
than in test function I am using meteorites array, but I get EXC error (object is nil)
What I am doing wrong? Can someone explain me? Thanks
func fetchData(completion: #escaping ([Meteor]?, APIError<Service.JSONError>?) -> ()) {
print("fetching")
let request: APIRequest<Meteorites, JSONError> = tron.request("")
request.perform(withSuccess: { (meteorites) in
completion(meteorites.meteorites, nil)
}) { (err) in
completion(nil, err)
}
}
In case Meteorites class
class Meteorites: JSONDecodable {
var meteorites: [Meteor]
required init(json: JSON) throws {
print("fetched")
var meteorites = [Meteor]()
for meteorJson in json.array! {
let dateString = meteorJson["year"]
if dateString != JSON.null {
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS" //Your date format
dateFormatter.timeZone = TimeZone(abbreviation: "GMT+0:00")!
let date = dateFormatter.date(from: dateString.stringValue)
if let compareDate = dateFormatter.date(from: "2011-01-01T00:00:00.000") {
if date! >= compareDate {
let meteor = Meteor()
meteor.date = date!
meteor.name = meteorJson["name"].stringValue
meteor.fall = meteorJson["fall"].stringValue
meteor.id = meteorJson["id"].intValue
meteor.reclong = meteorJson["reclong"].doubleValue
meteor.reclat = meteorJson["reclat"].doubleValue
meteor.mass = meteorJson["mass"].intValue
meteor.nametype = meteorJson["nametype"].stringValue
meteor.recclass = meteorJson["recclass"].stringValue
meteor.lastUpdate = NSDate()
meteorites.append(meteor)
}
}
}
}
self.meteorites = meteorites
}
}