What causes "parent cannot be null" in stride3d/stride?

Ensure undo operations are cleared when the asset/hierarchy is reloaded so stale operations are not replayed

stride3d/stride · error · InvalidOperationException

parent cannot be null

Error message

parent cannot be null

What it means

During Undo of a FolderDeleted operation, the code tries to locate the deleted folder's parent folder; if the parent reference is null it cannot recreate the folder and throws InvalidOperationException with 'parent cannot be null'.

Solutions

Ensure undo operations are cleared when the asset/hierarchy is reloaded so stale operations are not replayed
Check why the parent folder no longer exists (asset externally modified) and reconcile state before undo
Guard undo of FolderDeleted with a check that the parent path still resolves
Recreate the missing parent folders before replaying the operation

Example fix

// before
op.Undo(); // may throw if hierarchy reloaded
// after
if (op.ResolvesParent()) op.Undo(); else assetVm.ReloadOperations();

Defensive patterns

Strategy: try-catch

Validate before calling

if (parent == null || !asset.Folders.Contains(parent)) { RefreshHierarchy(); return; }

Type guard

static bool CanUndoFolderDelete(EntityFolderOperation op) => op.ResolvesParent();

Try / catch

try { op.Undo(); }
catch (InvalidOperationException ex) { log.Warn(ex); editor.RefreshHierarchy(); }

Prevention

Clear undo stacks on asset reload
Refresh hierarchy state before replaying old undo operations
Avoid external modifications to asset folders while undo history is active

When it happens

Trigger: Undoing a folder deletion after the hierarchy was reloaded/changed so the resolved parent (parent?.Folders...) is null and the folder cannot be recreated under it.

Common situations: Undo stacks surviving asset reloads or scene switches; deleting a root-level folder whose parent lookup returns null; editor state desynchronized from disk after external modifications.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/63af2629628f2bcf. Report an issue: GitHub.

Appendix: source

Thrown at sources/editor/Stride.Assets.Presentation/AssetEditors/EntityHierarchyEditor/ViewModels/EntityFolderOperation.cs:92

            {
                parent = parent?.Folders.FirstOrDefault(x => string.Equals(x.Name, folderPath[i], EntityFolderViewModel.FolderCase));
            }
            // Note: empty folders are not saved and can "disappear" after closing/reopening an editor.
            // In order for action to be undo/redo-able we must allow the possibility to not find the folder.
            EntityFolderViewModel folder;
            switch (action)
            {
                case Action.FolderCreated:
                    folder = parent?.Folders.FirstOrDefault(x => string.Equals(x.Name, folderName, EntityFolderViewModel.FolderCase));
                    folder?.Delete();
                    action = Action.FolderDeleted;
                    break;
                case Action.FolderDeleted:
                    folder = parent?.Folders.FirstOrDefault(x => string.Equals(x.Name, folderName, EntityFolderViewModel.FolderCase));
                    if (folder == null && editor != null)
                    {
                        // folder not found, let's recreate it
                        if (parent == null) throw new InvalidOperationException($"{nameof(parent)} cannot be null");
                        folder = editor.CreateFolder(asset, parent, false);
                        folder.Name = folderName;
                    }
                    action = Action.FolderCreated;
                    break;
                case Action.FolderRenamed:
                    folder = parent?.Folders.FirstOrDefault(x => string.Equals(x.Name, folderName, EntityFolderViewModel.FolderCase));
                    if (folder != null)
                    {
                        folder.Name = oldFolderName;
                    }
                    folderPath[folderPath.Length - 1] = oldFolderName;
                    oldFolderName = folderName;
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }
        }

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