Do not destroy the life cycle when the user is invalid.

components/proximity_auth/proximity_auth_system.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/proximity_auth/proximity_auth_system.h"
 
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/proximity_auth/logging/logging.h"
 #include "components/proximity_auth/proximity_auth_client.h"
 #include "components/proximity_auth/remote_device_life_cycle_impl.h"
@@ skipping 99 matching lines @@
 }
 
 void ProximityAuthSystem::OnLifeCycleStateChanged(
     RemoteDeviceLifeCycle::State old_state,
     RemoteDeviceLifeCycle::State new_state) {
   unlock_manager_->OnLifeCycleStateChanged();
 }
 
 void ProximityAuthSystem::OnScreenDidLock(
     ScreenlockBridge::LockHandler::ScreenType screen_type) {
-  OnFocusedUserChanged(ScreenlockBridge::Get()->focused_account_id());
+  const AccountId& focused_account_id =
+      ScreenlockBridge::Get()->focused_account_id();
+  if (focused_account_id.is_valid())
+    OnFocusedUserChanged(focused_account_id);
 }
 
 void ProximityAuthSystem::OnScreenDidUnlock(
     ScreenlockBridge::LockHandler::ScreenType screen_type) {
   unlock_manager_->SetRemoteDeviceLifeCycle(nullptr);
   remote_device_life_cycle_.reset();
 }
 
 void ProximityAuthSystem::OnFocusedUserChanged(const AccountId& account_id) {
   // Update the current RemoteDeviceLifeCycle to the focused user.
@@ skipping 24 matching lines @@
     PA_LOG(INFO) << "Creating RemoteDeviceLifeCycle for focused user: "
                  << account_id.Serialize();
     remote_device_life_cycle_ = CreateRemoteDeviceLifeCycle(remote_device);
     unlock_manager_->SetRemoteDeviceLifeCycle(remote_device_life_cycle_.get());
     remote_device_life_cycle_->AddObserver(this);
     remote_device_life_cycle_->Start();
   }
 }
 
 }  // proximity_auth