fix xr button state (#189)

Author: C. M. Barth

Apps/Playground/App.tsx

@@ -93,13 +93,15 @@ const EngineScreen: FunctionComponent<ViewProps> = (props: ViewProps) => {
     (async () => {
       if (xrSession) {
         await xrSession.exitXRAsync();
-        setXrSession(undefined);
-        setTrackingState(undefined);
       } else {
         if (rootNode !== undefined && scene !== undefined) {
           const xr = await scene.createDefaultXRExperienceAsync({ disableDefaultUI: true, disableTeleportation: true })
           const session = await xr.baseExperience.enterXRAsync("immersive-ar", "unbounded", xr.renderTarget);
           setXrSession(session);
+          session.onXRSessionEnded.add(() => {
+            setXrSession(undefined);
+            setTrackingState(undefined);
+          })
 
           setTrackingState(xr.baseExperience.camera.trackingState);
           xr.baseExperience.camera.onTrackingStateChanged.add((newTrackingState) => {