getTreeFromFlatData set rootKey as null

The function getTreeFromFlatData requires setting rootKey as null for it to work. However, the type of rootKey is set as string | number | undefined, resulting in a type error: Type 'null' is not assignable to type 'string | number | undefined'.ts(2322).

Can someone help to take a look?

code snippet:
```
const treeItems = await import("react-sortable-tree").then(treeLib => {
return treeLib.getTreeFromFlatData({
flatData,
getKey: node => node.id,
getParentKey: node => node.parentId,
// tslint:disable-next-line:no-null-keyword
rootKey: null // The value of the parent key when there is no parent (i.e., at root level)
});
```