How to reproduce:
1. Create a new project
2. Open Package Manager -> set it to show preview packages
3. Install latest Mobile Notifications Package
4. Open Notifications settings (you can click on it in Project NotificationSettings.asset or go to Editor -> Project Settings -> Mobile Notifications)
5. Switch from Android to iOS and back in settings
Result: notice a lot of errors in the console

NullReferenceException: Object reference not set to an instance of an object
Unity.Notifications.UnityNotificationEditorManager.SaveSetting (Unity.Notifications.NotificationEditorSetting setting, Unity.Notifications.NotificationEditorSettingsCollection values) (at Library/PackageCache/com.unity.mobile.notifications@1.0.0-preview.17/Editor/UnityNotificationEditorManager.cs:209)

GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced.

Tested and reproduced: 2018.3.13f1, 2019.1.0a12, 2019.1.0f2, 2019.2.0a12. Packages: preview.13-1.0.0, preview.17-1.0.0
Tested and didn't reproduce: 2019.2.0a12. Packages: preview.10-1.0.0