Can't add new entries to PackageDefinition

[youarebritish]

Attempting to do so throws the following error:

ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index System.ThrowHelper.ThrowArgumentOutOfRangeException (System.ExceptionArgument argument, System.ExceptionResource resource) (at :0) System.ThrowHelper.ThrowArgumentOutOfRangeException () (at :0) System.Collections.Generic.List`1[T].get_Item (System.Int32 index) (at :0) FoxKit.Modules.Archive.Importer.PackageDefinitionEditor.OnItemInserted (System.Object sender, Rotorz.Games.Collections.ItemInsertedEventArgs args) (at Assets/FoxKit/Modules/Package/Editor/PackageDefinitionEditor.cs:51) Rotorz.Games.Collections.ReorderableListControl.OnItemInserted (Rotorz.Games.Collections.ItemInsertedEventArgs args) (at Assets/Lib/unity3d-reorderable-list/Editor/Collections/ReorderableListControl.cs:465) Rotorz.Games.Collections.ReorderableListControl.AddItem (Rotorz.Games.Collections.IReorderableListAdaptor adaptor) (at Assets/Lib/unity3d-reorderable-list/Editor/Collections/ReorderableListControl.cs:1965) Rotorz.Games.Collections.ReorderableListControl.DrawFooterControls (UnityEngine.Rect position, Rotorz.Games.Collections.IReorderableListAdaptor adaptor) (at Assets/Lib/unity3d-reorderable-list/Editor/Collections/ReorderableListControl.cs:1266) Rotorz.Games.Collections.ReorderableListControl.Draw (System.Int32 controlID, Rotorz.Games.Collections.IReorderableListAdaptor adaptor, Rotorz.Games.Collections.ReorderableListControl+DrawEmpty drawEmpty) (at Assets/Lib/unity3d-reorderable-list/Editor/Collections/ReorderableListControl.cs:1455) Rotorz.Games.Collections.ReorderableListControl.Draw (Rotorz.Games.Collections.IReorderableListAdaptor adaptor) (at Assets/Lib/unity3d-reorderable-list/Editor/Collections/ReorderableListControl.cs:1469) FoxKit.Modules.Archive.Importer.PackageDefinitionEditor.OnInspectorGUI () (at Assets/FoxKit/Modules/Package/Editor/PackageDefinitionEditor.cs:92) UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, System.Int32 editorIndex, System.Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1367) UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)