FIX: Exception when clicking 'Add Action' in empty asset (case 1199562, #1034).

Author: Rene Damm

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -15,6 +15,7 @@ however, it has to be formatted properly to pass verification tests.
 
 #### Actions
 
+- When no action map is selected in action editor, plus icon to add an action is now disabled; formerly threw an exception when clicked (case 1199562).
 - Removing a callback from actions from the callback itself no longer throws `ArgumentOutOfRangeException` ([case 1192972](https://issuetracker.unity3d.com/issues/input-system-package-argumentoutofrangeexception-error-is-thrown-when-the-callback-is-removed-while-its-being-triggered)).
 - "Invalid user" `ArgumentException` when turning the same `PlayerInput` on and off ([case 1198889](https://issuetracker.unity3d.com/issues/input-system-package-argumentexception-invalid-user-error-is-thrown-when-the-callback-disables-game-object-with-playerinput)).
 - The list of device requirements for a control scheme in the action editor no longer displays devices with their internal layout name rather than their external display name.

Packages/com.unity.inputsystem/InputSystem/Editor/AssetEditor/InputActionEditorWindow.cs

@@ -449,7 +449,7 @@ private void RebuildActionTree()
             if (selectedActionMapItem == null)
             {
                 // Nothing selected. Wipe middle and right pane.
-                m_ActionsTree.onBuildTree = () => new TreeViewItem(0, -1, "");
+                m_ActionsTree.onBuildTree = null;
             }
             else
             {