FIX: Give an appropriate error message if an asset went missing (ISX-1921) (#1881)

* FIX: Give an appropriate error message if an asset went missing (ISX-1921)

* Fix format

* Fix ArgumentException discovered during testing

* Pre-existing issue and unrelated to original change; occurs when asset's .meta file deleted but not actual asset

* If we don't have a working copy of the asset on reload, we need to create one.

---------

Co-authored-by: Tim Keosababian <36088732+timkeo@users.noreply.github.com>
Co-authored-by: Håkan Sidenvall <hakan.sidenvall@unity3d.com>

Author: 3 people

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

@@ -198,7 +198,7 @@ internal void SaveChangesToAsset()
             // If this is invoked after a domain reload, importAsset will resolve itself.
             // However, if the asset do not exist importedAsset will be null and we cannot complete the operation.
             if (importedAsset == null)
-                throw new Exception("Unable to save changes. Associated asset do not exist.");
+                throw new Exception("Unable to save changes. Associated asset does not exist.");
 
             SaveAsset(path, m_AssetObjectForEditing.ToJson());
             SetDirty(false);

Packages/com.unity.inputsystem/InputSystem/Editor/UITKAssetEditor/InputActionsEditorWindow.cs

@@ -165,17 +165,32 @@ private void CreateGUI() // Only domain reload
             // Therefore we recreate the state here using the fields which were saved.
             if (m_State.serializedObject == null)
             {
+                InputActionAsset workingCopy = null;
                 try
                 {
                     var assetPath = AssetDatabase.GUIDToAssetPath(m_AssetGUID);
                     var asset = AssetDatabase.LoadAssetAtPath<InputActionAsset>(assetPath);
+
+                    if (asset == null)
+                        throw new Exception($"Failed to load asset \"{assetPath}\". The file may have been deleted or moved.");
+
                     m_AssetJson = InputActionsEditorWindowUtils.ToJsonWithoutName(asset);
-                    m_State = new InputActionsEditorState(m_State, new SerializedObject(m_AssetObjectForEditing));
+
+                    if (m_AssetObjectForEditing == null)
+                    {
+                        workingCopy = InputActionAssetManager.CreateWorkingCopy(asset);
+                        m_State = new InputActionsEditorState(m_State, new SerializedObject(workingCopy));
+                        m_AssetObjectForEditing = workingCopy;
+                    }
+                    else
+                        m_State = new InputActionsEditorState(m_State, new SerializedObject(m_AssetObjectForEditing));
                     m_IsDirty = HasContentChanged();
                 }
                 catch (Exception e)
                 {
                     Debug.LogException(e);
+                    if (workingCopy != null)
+                        DestroyImmediate(workingCopy);
                     Close();
                     return;
                 }
@@ -316,7 +331,7 @@ private void OnDestroy()
             if (m_AssetObjectForEditing != null)
                 DestroyImmediate(m_AssetObjectForEditing);
 
-            m_View.DestroyView();
+            m_View?.DestroyView();
         }
 
         private void ReshowEditorWindowWithUnsavedChanges()