NoSuchMethodError: The method 'containsKey' was called on null (.Easy Localization)

[MuhammadSufyanMalik]

if I run on the ios platform I got the below error, whereas there is no error on the android platform.
Below is the console output of the error.

Launching lib/main.dart on iPod touch (7th generation) in debug mode...
lib/main.dart:1
Xcode build done. 34.9s
Connecting to VM Service at ws://127.0.0.1:51398/Yrw7tsntuls=/ws
[Easy Localization] Start
[Easy Localization] Init state
[Easy Localization] Build
[Easy Localization] Device locale en
[Easy Localization] Init Localization Delegate
[Easy Localization] Init provider
[Easy Localization] Load Localization Delegate

════════ Exception caught by widgets library ═══════════════════════════════════
The following NoSuchMethodError was thrown building Body(dirty, dependencies: [MediaQuery]):
The method 'containsKey' was called on null.
Receiver: null
Tried calling: containsKey("parent_btn")

The relevant error-causing widget was
Body
lib/…/registration/RegistrationOptions.dart:9
When the exception was thrown, this was the stack
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
#1 Translations.isNestedKey
package:easy_localization/src/translations.dart:40
#2 Translations.get
package:easy_localization/src/translations.dart:7
#3 Localization._resolve
package:easy_localization/src/localization.dart:138
#4 Localization.tr
package:easy_localization/src/localization.dart:46
...

[MuhammadSufyanMalik]

Error resolved, by adding Languages json separately.