iCloud Drive Issue: "[DocumentManager] Failed to associate thumbnails for picked URL"

Question

I've created a JSON string file from an object containing multiple properties. This is the object:

RecipeFile : Codable {
  var name: String
  var theRecipeIngredients: [String]
  var theRecipeSteps: [String]
  var theRecipeRating: Int
  var theRecipeCategory: String
  var theRecipeIndexStrings: String
  var theRecipeImage: String?

I create the JSON string file with this code:

let json_encoder = JSONEncoder()
let recipeFileName = recipeToDisplay.name! + UUID().uuidString + ".json"
let exportFilePath = getDocumentsDirectory().appendingPathComponent(recipeFileName)
do {
   let jsonData = try json_encoder.encode(exportRecipeFile)
   if let jsonString = String(data: jsonData, encoding: .utf8)
      {
         try jsonString.write(to: exportFilePath, atomically: false, encoding: .utf8)
      }
   } catch {
      print(error.localizedDescription)
   }

I upload it to iCloud Drive. I import the string file from iCloud Drive using UIDocumentPickerViewController. I can parse the imported file just fine. However, I get this message (edited to remove some path info) in the xCode debug area when func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentsAt urls: [URL]) is called:

[DocumentManager] Failed to associate thumbnails for picked URL file:///....Bourbon%20Chocolate%20Walnut%20Pie18D20181-DAFD-499C-9873-7D3E0794C37A.json with the Inbox copy file:///....Bourbon%20Chocolate%20Walnut%20Pie18D20181-DAFD-499C-9873-7D3E0794C37A.json: Error Domain=QLThumbnail Code=2 "(null)" UserInfo={NSUnderlyingError=0x149a042b0 {Error Domain=GSLibraryErrorDomain Code=3 "Generation not found" UserInfo={NSDescription=Generation not found}}}

Any idea what is causing this to be generated?

The didPickDocumentsAt code starts as follows:

    let data = try? Data(contentsOf: urls[0]) as Data
    let json_decoder = JSONDecoder()
    do {
        let importRecipeFile = try json_decoder.decode(RecipeFile.self, from: data!)
        let importedRecipeToSave = Recipe(context: theMOC)
        importedRecipeToSave.name = importRecipeFile.name
        importedRecipeToSave.category = importRecipeFile.theRecipeCategory
        importedRecipeToSave.rating = Int16(importRecipeFile.theRecipeRating)
        importedRecipeToSave.terms = importRecipeFile.theRecipeIndexStrings
        importedRecipeToSave.addedToGroceryList = false

Answer 1

You can safely ignore this message. When you import a file from iCloud, iOS tries to copy the thumbnail from iCloud to the imported copy, but for JSON files there's no thumbnail to copy and it logs this. This is not an error on your side.