Fix type error

Author: clauderic

packages/react/src/core/draggable/DragOverlay.tsx

@@ -11,7 +11,7 @@ export interface Props<T extends Data, U extends Draggable<T>> {
   children: ReactNode | ((source: U) => ReactNode);
   style?: React.CSSProperties;
   tag?: string;
-  disabled?: boolean | ((source: U) => boolean);
+  disabled?: boolean | ((source: U | null) => boolean);
 }
 
 export function DragOverlay<T extends Data, U extends Draggable<T>>({
@@ -23,10 +23,8 @@ export function DragOverlay<T extends Data, U extends Draggable<T>>({
 }: Props<T, U>) {
   const ref = useRef<HTMLDivElement | null>(null);
   const manager = useDragDropManager();
-  const source = useComputed(
-    () => manager?.dragOperation.source,
-    [manager]
-  ).value;
+  const source: U | null =
+    useComputed(() => manager?.dragOperation.source, [manager]).value ?? null;
   const isDisabled =
     typeof disabled === 'function' ? disabled(source) : disabled;