Fix vhscore workflow undefined error

Author: fichas

web/js/VHS.core.js

@@ -2143,6 +2143,9 @@ app.registerExtension({
         let originalGraphToPrompt = app.graphToPrompt
         let graphToPrompt = async function() {
             let res = await originalGraphToPrompt.apply(this, arguments);
+            if (!res || !res.workflow) {
+                return res;
+            }
             res.workflow.extra['VHS_latentpreview'] = app.ui.settings.getSettingValue("VHS.LatentPreview")
             res.workflow.extra['VHS_latentpreviewrate'] = app.ui.settings.getSettingValue("VHS.LatentPreviewRate")
             res.workflow.extra['VHS_MetadataImage'] = app.ui.settings.getSettingValue("VHS.MetadataImage")