Coordinate file read in resolveRelativePath

Otherwise we can't get a bookmark

Author: amake

ios/Classes/SwiftFilePickerWritablePlugin.swift

@@ -173,8 +173,24 @@ public class SwiftFilePickerWritablePlugin: NSObject, FlutterPlugin {
         }
         let childUrl = url.appendingPathComponent(relativePath).standardized
         logDebug("Resolved to \(childUrl)")
+        var coordError: NSError? = nil
+        var bookmarkError: Error? = nil
+        var identifier: String? = nil
+        // Coordinate reading the item here because it might be a
+        // not-yet-downloaded file, in which case we can't get a bookmark for
+        // it--bookmarkData() fails with a "file doesn't exist" error
+        NSFileCoordinator().coordinate(readingItemAt: childUrl, error: &coordError) { url in
+            do {
+                identifier = try childUrl.bookmarkData().base64EncodedString()
+            } catch let error {
+                bookmarkError = error
+            }
+        }
+        if let error = coordError ?? bookmarkError {
+            throw error
+        }
         result([
-            "identifier": try childUrl.bookmarkData().base64EncodedString(),
+            "identifier": identifier,
             "persistable": "true",
             "uri": childUrl.absoluteString,
             "fileName": childUrl.lastPathComponent,