Missing the class attribute 'ExtensionOfNativeClass'

After updating unity to the version 2018.3.7f1 I have tihs error in my released project:

 'Saves' is missing the class attribute 'ExtensionOfNativeClass'!
 'SavesForAllSlots' is missing the class attribute 'ExtensionOfNativeClass'!
 'Saves' is missing the class attribute 'ExtensionOfNativeClass'!
 UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
 'SavesForAllSlots' is missing the class attribute 'ExtensionOfNativeClass'!
 UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
 ...

Why is tihs happening?

Saves.cs

 [System.Serializable]
 public class Saves
 {
     public string dateTime;
     public float transformPositionX;
     public float transformPositionY;
     public float transformPositionZ;
     public int latestSaveSlot;
     public int actNumber;
     public string sceneName;
     public int slotImage;
     public int currentActiveSlot;
     public int stepNumber;
 }

SavesForAllSlots.cs

 [System.Serializable]
 public class SavesForAllSlots
 {
     public int latestSaveSlotForAll;
 }