fix: implement layout delegate interface

Author: nwidynski

packages/@react-stately/virtualizer/src/Layout.ts

@@ -63,7 +63,7 @@ export abstract class Layout<T extends object = Node<any>, O = any> implements L
   /**
    * Returns the orientation of the layout.
    */
-  getOrientation(): Orientation {
+  getOrientation?(): Orientation | null {
     return this.orientation;
   }
 

packages/@react-stately/virtualizer/src/Virtualizer.ts

@@ -191,7 +191,7 @@ export class Virtualizer<T extends object, V> {
     if (isTestEnv && !(isClientWidthMocked && isClientHeightMocked)) {
       rect = new Rect(0, 0, this.contentSize.width, this.contentSize.height);
     } else {
-      rect = this._overscanManager.getOverscannedRect(this.layout.getOrientation());
+      rect = this._overscanManager.getOverscannedRect(this.layout.getOrientation!()!);
     }
     let layoutInfos = this.layout.getVisibleLayoutInfos(rect);
     let map = new Map;