DispatchGroup with SQLite database

I'm trying to get my head around GCD, specifically DispatchGroup to organise downloads to a SQLite database via the FMDB wrapper. My app does the following:

• Downloads info on available subjects at app startup from remote server with SQL db. Saves these locally in SQLite db for future sessions and presents what's available via UITableViewController
• If a subject is selected, its contents are downloaded from the server and saved locally for future sessions. I do it this way rather than all at once at startup as this is a precursor to in-app purchases. I also download some other stuff here. Then segue to new tableview of subject contents.
• I can achieve the above by chaining the download & save functions together with completion handlers, however I'd like to make use of DispatchGroup so I can utilise wait(timeout:) function in the future.

However, with my implementation of DispatchGroup (below) I'm receiving the following errors.

API call with NULL database connection pointer
[logging] misuse at line 125820 of [378230ae7f]

And also

BUG IN CLIENT OF libsqlite3.dylib: illegal multi-threaded access to database connection

Code as follows:

didSelectRow

override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

//Download from server
if availableSubjects[indexPath.row].isDownloaded == 0 {
    //CHAINING THIS WAY WORKS
    /* downloadModel.downloadCaseBundle(withSubjectID: indexPath.row, completion: {
       self.downloadModel.downloadToken(forSubject: indexPath.row, completion: {
       self.caseBundle = DBManager.sharedDBManager.getCaseBundle(forSubject: indexPath.row)
       self.availableSubjects[indexPath.row].isDownloaded = 1
       DispatchQueue.main.async {
           self.performSegue(withIdentifier: "showCaseList", sender: self)
           }
       })
  })*/

    let dispatchGroup = DispatchGroup()

    //Download content
    dispatchGroup.enter()
    downloadModel.downloadCaseBundle(withSubjectID: indexPath.row) {
        dispatchGroup.leave()
    }

    //Download token
    dispatchGroup.enter()
    downloadModel.downloadToken(forSubject: indexPath.row) {
        dispatchGroup.leave()
    }

    //Execute
    dispatchGroup.notify(queue: .main) {
    self.caseBundle = DBManager.sharedDBManager.getCaseBundle(forSubject: indexPath.row)
    self.availableSubjects[indexPath.row].isDownloaded = 1
    self.performSegue(withIdentifier: "showCaseList", sender: self)
    }

} else { //Already downloaded, just retrieve from local db and present
    caseBundle = DBManager.sharedDBManager.getCaseBundle(forSubject: indexPath.row)
    self.performSegue(withIdentifier: "showCaseList", sender: self)
    }       
}

DownloadModel, downloadCaseBundle

downloadToken function is more or less identical

func downloadCaseBundle(withSubjectID subjectID: Int, completion: @escaping () -> Void) {
    let urlPath = "someStringtoRemoteDB"
    let url: URL = URL(string: urlPath)!
    let defaultSession = Foundation.URLSession(configuration: URLSessionConfiguration.default)

    let task = defaultSession.dataTask(with: url) { (data, response, error) in

        if error != nil {
            print("Error")
        } else {
            print("cases downloaded")
            self.parseCasesJSON(data!, header: self.remoteMasterTable, forSubject: subjectID)
            completion()
        }
    }
    task.resume()
}

Download Mode, parseJSON

func parseCasesJSON(_ data:Data, header: String, forSubject subjectID: Int) {
        var jsonResult = NSArray()
        var jsonElement = NSDictionary()
        let cases = NSMutableArray()

        do {
            jsonResult = try JSONSerialization.jsonObject(with: data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSArray
        } catch let error as NSError {
            print(error)
            print("error at serialisation")
        }

        //Iterate through JSON result (i.e. case), construct and append to cases array
        for i in 0 ..< jsonResult.count {
            jsonElement = jsonResult[i] as! NSDictionary
            var caseObject = CaseModel()

            //The following insures none of the JsonElement values are nil through optional binding
            if let uniqueID = jsonElement["id"] as? Int,
                let subjectTitle = jsonElement["subjectTitle"] as? String,
                let subjectID = jsonElement["subjectID"] as? Int,
                let questionID = jsonElement["questionID"] as? Int,
                //And so on
            {
                caseObject.uniqueID = uniqueID
                caseObject.subjectTitle = subjectTitle
                caseObject.subjectID = subjectID
                caseObject.questionID = questionID
                //And so on
            }
            cases.add(caseObject)
        }

        DBManager.sharedDBManager.saveCasesLocally(dataToSave: cases as! [CaseModel])
        DBManager.sharedDBManager.setSubjectAsDownloaded(forSubjectID: subjectID)
    }

Turns out it was nothing to do with those methods and I needed to implement FMDatabaseQueue instead of FMDatabase in my DBManager singleton.