When a higher-priority xDS server responds while a lower-priority fallback ControlPlaneClient already has a DiscoveryResponse runnable queued in the shared SynchronizationContext, calling shutdownLowerPriorityCpcs() removes the fallback entry from serverLrsClientMap and invokes cpc.shutdown().

Because ControlPlaneClient.shutdown() previously scheduled its cleanup via syncContext.execute(...), the cleanup was placed at the tail of the queue behind the already-queued response runnable. When that queued response runnable executed, adsStream had not been closed yet, so handleResourceUpdate() looked up serverLrsClientMap.get(serverInfo) (now null) and threw a NullPointerException when calling controlPlaneClient.ackResponse(...).

Fix this by:

1. Executing ControlPlaneClient.shutdown() directly on syncContext (enforcing syncContext.throwIfNotInThisSynchronizationContext()), which all callers already satisfy, so adsStream is closed immediately before any subsequently queued runnables run.
2. Adding a defensive null check for controlPlaneClient in XdsClientImpl.handleResourceUpdate().

more context: http://b/469140342