remove null as unknown

Author: devjiwonchoi

packages/next/src/next-devtools/dev-overlay/components/devtools-panel/devtools-panel.tsx

@@ -32,7 +32,7 @@ export function DevToolsPanel({
   dispatch: OverlayDispatch
 }) {
   const [position, setPosition] = useState<Corners>(getInitialPosition())
-  const dialogRef = useRef<HTMLDivElement>(null as unknown as HTMLDivElement)
+  const dialogRef = useRef<HTMLDivElement>(null)
 
   // This hook lets us do an exit animation before unmounting the component
   const { mounted, rendered } = useDelayedRender(state.isDevToolsPanelOpen)