FIX: ISXB-1153 fix exception on paste to empty actionmap list

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -20,6 +20,7 @@ however, it has to be formatted properly to pass verification tests.
 - Fixed unexpected control scheme switch when using `OnScreenControl` and pointer based schemes which registed "Cancel" event on every frame.[ISXB-656](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-656).
 - Fixed an issue with The "Add Control Scheme..." popup window so that it now persists until any changes are explicitly Saved or Cancelled [case ISXB-1131](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1131).
 - Fixed missing documentation for source generated Input Action Assets. This is now generated as part of the source code generation step when "Generate C# Class" is checked in the importer inspector settings.
+- Fixed pasting into an empty map list raising an exception. [ISXB-1150](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1150)
 
 ### Changed
 - Added back the InputManager to InputSystem project-wide asset migration code with performance improvement (ISX-2086).

Packages/com.unity.inputsystem/InputSystem/Editor/UITKAssetEditor/Views/CopyPasteHelper.cs

@@ -243,6 +243,9 @@ internal static void PasteItems(string copyBufferString, int[] indicesToInsert,
             // Split buffer into transmissions and then into transmission blocks
             var copiedType = GetCopiedType(copyBufferString);
             int indexOffset = 0;
+            // If the array is empty, make sure we insert at index 0
+            if (arrayToInsertInto.arraySize == 0)
+                indexOffset = -1;
             foreach (var transmission in copyBufferString.Substring(k_CopyPasteMarker.Length + k_TypeMarker[copiedType].Length)
                      .Split(new[] {k_EndOfTransmission}, StringSplitOptions.RemoveEmptyEntries))
             {

Packages/com.unity.inputsystem/InputSystem/Editor/UITKAssetEditor/Views/Selectors.cs

@@ -114,7 +114,12 @@ public static int GetSelectedBindingIndexAfterCompositeBindings(InputActionsEdit
 
         public static int GetBindingIndexBeforeAction(SerializedProperty arrayProperty, int indexToInsert, SerializedProperty bindingArrayToInsertTo)
         {
-            Debug.Assert(indexToInsert >= 0 && indexToInsert <= arrayProperty.arraySize, "Invalid action index to insert bindings before.");
+            // Need to guard against this case, as there is different behaviour when pasting actions vs actionmaps
+            if (indexToInsert < 0)
+            {
+                return -1;
+            }
+            Debug.Assert(indexToInsert <= arrayProperty.arraySize, "Invalid action index to insert bindings before.");
             var offset = 1; //previous action offset
             while (indexToInsert - offset >= 0)
             {