How to solve "parallel routes missing default page" error in Next.js 16?

app/dashboard/
├── layout.tsx
├── page.tsx
├── @sidebar/
│   ├── page.tsx
│   └── default.tsx    ← Required
└── @main/
    ├── page.tsx
    └── default.tsx    ← Required

// app/dashboard/@sidebar/default.tsx
export default function DefaultSidebar() {
  return <div>Select a sidebar view</div>;
}

// app/dashboard/@main/default.tsx
export default function DefaultMain() {
  return null;
}

// app/dashboard/layout.tsx
export default function Layout({ children, sidebar }: { 
  children: React.ReactNode; 
  sidebar?: React.ReactNode 
}) {
  return (
    <div style={{ display: 'flex' }}>
      <main>{children}</main>
      <aside>{sidebar}</aside>
    </div>
  );
}