LykosAI/StabilityMatrix · error · ValidationException

Scheduler not selected

Error message

Scheduler not selected

What it means

WanSamplerCardViewModel.ApplyStep() reads SelectedScheduler (nullable ComfyScheduler?) and assigns it as the workflow's PrimaryScheduler. When no scheduler is selected it throws a ValidationException("Scheduler not selected") to abort building the ComfyUI graph. This is the scheduler counterpart of the 'Sampler not selected' check on the same card.

Solutions

1. Connect to ComfyUI so the scheduler list loads, then select a scheduler in the Wan sampler card before generating.
2. Restart the Inference tab or the app — the constructor defaults SelectedScheduler to ComfyScheduler.Simple, so a fresh card resolves the null.
3. If instantiating the card in code/tests, set SelectedScheduler explicitly (e.g. card.SelectedScheduler = ComfyScheduler.Simple) before ApplyStep.
4. Verify the scheduler dropdown's ItemsSource binding is populated from the client manager's scheduler list.

Example fix

// before (throws)
var primaryScheduler = SelectedScheduler ?? throw new ValidationException("Scheduler not selected");
// after (defensive default)
var primaryScheduler = SelectedScheduler ?? ComfyScheduler.Simple;

if (card.SelectedScheduler is null)
    throw new ValidationException("Select a scheduler before generating.");
card.ApplyStep(e);

bool HasScheduler(WanSamplerCardViewModel c) => c.SelectedScheduler is not null;

try { card.ApplyStep(e); }
catch (ValidationException ex) { NotifyUser(ex.Message); }

When it happens

Trigger: Running a Wan workflow when SelectedScheduler is null at ApplyStep time — the scheduler dropdown was never populated (ComfyUI disconnected / scheduler list not loaded), the selection was cleared, or the card was constructed manually without setting SelectedScheduler.

Common situations: ComfyUI backend not connected so schedulers never load; a restored prompt/layout carries a scheduler value that no longer resolves, leaving null; unit tests or other code paths instantiating WanSamplerCardViewModel via DI and invoking ApplyStep directly; binding failure between the scheduler combo box and the view model.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Sampler not selected
• : Model not selected
• BaseClipVision not set
• Prompt extensions not installed
• Resources.Validation_PackageNameCannotBeEmpty

AI-assisted analysis of LykosAI/StabilityMatrix@af93d6ef57 (2026-09-12). Data as JSON: /api/errors/89ccad5c2d6bc928. Report an issue: GitHub.

Appendix: source

Thrown at StabilityMatrix.Avalonia/ViewModels/Inference/WanSamplerCardViewModel.cs:51

        SelectedScheduler = ComfyScheduler.Simple;
        Length = 33;
    }

    public override void ApplyStep(ModuleApplyStepEventArgs e)
    {
        if (EnableAddons)
        {
            foreach (var module in ModulesCardViewModel.Cards.OfType<ModuleBase>())
            {
                module.ApplyStep(e);
            }
        }

        // Set primary sampler and scheduler
        var primarySampler = SelectedSampler ?? throw new ValidationException("Sampler not selected");
        e.Builder.Connections.PrimarySampler = primarySampler;

        var primaryScheduler = SelectedScheduler ?? throw new ValidationException("Scheduler not selected");
        e.Builder.Connections.PrimaryScheduler = primaryScheduler;

        // for later inheritance if needed
        e.Builder.Connections.PrimaryCfg = CfgScale;
        e.Builder.Connections.PrimarySteps = Steps;
        e.Builder.Connections.PrimaryModelType = SelectedModelType;

        e.Temp = e.CreateTempFromBuilder();

        var conditioning = e.Temp.Base.Conditioning.Unwrap();

        var isImgToVid = IsDenoiseStrengthEnabled;

        if (isImgToVid)
        {
            var clipVisionEncode = e.Nodes.AddTypedNode(
                new ComfyNodeBuilder.CLIPVisionEncode
                {

View on GitHub (pinned to af93d6ef57)