fix(workflows): keep the version history tab alive without a cascading workspace

[RalfvandenBurg]

The problem

VersionHistoryTab takes its workspace as a [CascadingParameter] of the concrete WorkflowDefinitionWorkspace type:

[CascadingParameter] private WorkflowDefinitionWorkspace Workspace { get; set; } = default!;

protected override void OnInitialized() => Workspace.WorkflowDefinitionUpdated += OnWorkflowDefinitionUpdated;
void IDisposable.Dispose()              => Workspace.WorkflowDefinitionUpdated -= OnWorkflowDefinitionUpdated;

Any host that cascades a workspace of a different type leaves that parameter null. Downstream hosts reach this state by copying WorkflowDefinitionWorkspace into their own namespace to change which WorkflowEditor it instantiates: the copy still does <CascadingValue Value="this"> and still renders this package's un-forked WorkflowProperties, but the cascaded value no longer matches the type this tab asks for.

Because both dereferences were unconditional, opening the tab produced two exceptions:

[Warning] ErrorBoundary   Unhandled exception rendering component: Object reference not set to an instance of an object.
   at ...Tabs.VersionHistory.VersionHistoryTab.OnInitialized()

[Warning] RemoteRenderer  Unhandled exception rendering component: Object reference not set to an instance of an object.
   at ...Tabs.VersionHistory.VersionHistoryTab.System.IDisposable.Dispose()
   at Microsoft.AspNetCore.Components.Rendering.ComponentState.DisposeInBatchAsync(RenderBatchBuilder)

[Error]   CircuitHost     Unhandled exception in circuit '...'

The first is caught by an error boundary. The second is thrown from Dispose() inside the renderer's disposal batch, where no boundary can catch it — so it terminates the entire Blazor circuit. The user sees An error has occurred. This application may no longer respond until reloaded. and every other page in the Studio is dead until they reload, not just this tab.

Version history is the only one of the four workflow property tabs that reaches for the workspace this way — Properties, Variables and Input/Output take the definition as an ordinary parameter — which is why nothing else broke.

The change

Workspace becomes nullable and every use tolerates its absence:

• OnInitialized / Dispose — subscribe and unsubscribe only when a workspace is present, so nothing can throw during the disposal batch.
• IsReadOnly → true and HasWorkflowEditPermission → false when there is no workspace, which disables bulk actions, rollback and delete through the existing CanRollback / CanDelete checks.
• ViewVersionAsync, OnRollbackClicked, OnDeleteClicked, OnBulkDeleteClicked — return early rather than dereference.

The version list itself still loads and is browsable, since it only needs DefinitionId and IWorkflowDefinitionService. The one behaviour given up is the auto-refresh subscription, and only in hosts where the parameter was already null.

This is deliberately the conservative fix. The underlying design question — that this tab depends on a concrete component type while its siblings depend on IWorkspace, which today carries only IsReadOnly and HasWorkflowEditPermission — would need IWorkspace widened with WorkflowDefinitionUpdated, DisplayWorkflowDefinitionVersionAsync, DisplayLatestWorkflowDefinitionVersionAsync, IsSelectedDefinition and GetSelectedDefinition. Happy to follow up with that if you would prefer it over the guards.

Tests

VersionHistoryTabMissingWorkspaceTests renders the tab with no cascading workspace and pins three things: the version list loads, disposal does not throw, and the edit actions are disabled. All three fail on main with NullReferenceException at VersionHistoryTab.OnInitialized() and pass with this change.

Elsa.Studio.Workflows.Tests: 210/210 pass. Elsa.Studio.Workflows builds with 0 warnings.

🤖 Generated with Claude Code

[greptile-apps]

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