[fix] Prevent error when deleting nodes with missing definitions

Adds defensive array access checks in widgetInputs.ts to handle cases where
customSpec is not a valid array when merging widget configurations. This
resolves the 'Cannot read properties of null (reading "1")' error that
occurs when attempting to delete nodes whose definitions have been removed.

Fixes #8338

src/extensions/core/widgetInputs.ts

@@ -506,7 +506,12 @@ export function mergeIfValid(
     }
   }
 
-  return { customConfig: customSpec?.[1] ?? {} }
+  return {
+    customConfig:
+      customSpec && Array.isArray(customSpec) && customSpec[1]
+        ? customSpec[1]
+        : {}
+  }
 }
 
 app.registerExtension({