docs: document when-then name

www/apps/resources/app/troubleshooting/workflow-errors/page.mdx

@@ -0,0 +1,24 @@
+export const metadata = {
+  title: `Workflow Errors`,
+}
+
+# {metadata.title}
+
+## When-Then Error: Handler for action X Not Found
+
+The following error may occur in production if you use a `when-then` block in your workflow:
+
+```plain
+custom-workflow:when-then-01JE8Z0M1FXSE2NCK1G04S0RR2:invoke - Handler for action \"when-then-01JE8Z0M1FXSE2NCK1G04S0RR2\" not found...
+```
+
+This occurs if the `when-then` block doesn't return a step's result and doesn't have a name specified. You can resolve it by passing a name as a first parameter of `when`:
+
+```ts
+const result = when(
+  "custom-when-condition",
+  // ... rest of the parameters
+)
+```
+
+Learn more about passing a name for `when-then` in [this documentation](!docs!/learn/fundamentals/workflows/conditions#specify-name-for-when-then)