On removing an item from the "Notification icons" list of "Mobile Notification Settings" preset causes editor to throw NullReferenceException continuously, refer the attached video.

Steps to repro:
1. Create a new project.
2. Window > Package Manager > Mobile Notifications (preview.5 - 1.0.4)
3. Edit > Project Settings > Mobile Notification Settings
4. Add icon in the "Notification icon" list
5. Create preset of Mobile Notification Settings
6. Project > UnityNotificationEditorManager preset
7. Inspector > Remove notification icons from the list

Actual Result:
NullReferenceException is thrown on removing an icon from the "Notification icons" list

Expected Result:
"Notification icons" get removed from the list without throwing any exception

Occurring in:
2020.1.0a15, 2019.3.0f2

Package version found: preview.5 - 1.0.4, 1.0.3, 1.0.2, 1.0.0

Environment:
Windows 10 & macOS 10.14.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
Unity.Notifications.UnityNotificationsEditorManagerEditor.DrawIconDataElement (UnityEngine.Rect rect, System.Int32 index, System.Boolean selected, System.Boolean focused) (at Library/PackageCache/com.unity.mobile.notifications@1.0.4-preview.5/Editor/UnityNotificationsEditorManagerEditor.cs:226)
Unity.Notifications.UnityNotificationsEditorManagerEditor.<OnEnable>b__19_5 (UnityEngine.Rect rect, System.Int32 index, System.Boolean selected, System.Boolean focused) (at Library/PackageCache/com.unity.mobile.notifications@1.0.4-preview.5/Editor/UnityNotificationsEditorManagerEditor.cs:140)
UnityEditorInternal.ReorderableList.DoListElements (UnityEngine.Rect listRect) (at <bbd1748017cd4e8b8384484cbd0cbbf2>:0)
UnityEditorInternal.ReorderableList.DoList (UnityEngine.Rect rect) (at <bbd1748017cd4e8b8384484cbd0cbbf2>:0)
Unity.Notifications.UnityNotificationsEditorManagerEditor.OnInspectorGUI (UnityEngine.Rect rect, System.Boolean drawInInspector) (at Library/PackageCache/com.unity.mobile.notifications@1.0.4-preview.5/Editor/UnityNotificationsEditorManagerEditor.cs:419)
Unity.Notifications.UnityNotificationsEditorManagerEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.mobile.notifications@1.0.4-preview.5/Editor/UnityNotificationsEditorManagerEditor.cs:341)
UnityEditor.Presets.PresetEditor.DrawInternalInspector () (at <bbd1748017cd4e8b8384484cbd0cbbf2>:0)
UnityEditor.Presets.PresetEditor.DrawPresetData () (at <bbd1748017cd4e8b8384484cbd0cbbf2>:0)
UnityEditor.Presets.PresetEditor.OnInspectorGUI () (at <bbd1748017cd4e8b8384484cbd0cbbf2>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <bbd1748017cd4e8b8384484cbd0cbbf2>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)