Add null check before invoking onChallengeInterruptFinished

It's possible that the previous client's owner has died (binderDied),
which results in ClientMonitor clearing the listener.

Fixes: 171189072
Test: Start enrollment, kill settings, enter device credential.
      No crash observed.
Change-Id: I09f5d206b1641e7c4bc1cb0b8ec22045e9c9d35f

services/core/java/com/android/server/biometrics/sensors/face/Face10.java

@@ -582,13 +582,19 @@ class Face10 implements IHwBinder.DeathRecipient {
                     final FaceGenerateChallengeClient previousChallengeOwner =
                             mCurrentChallengeOwner.getInterruptedClient();
                     mCurrentChallengeOwner = null;
+
                     Slog.d(TAG, "Previous challenge owner: " + previousChallengeOwner);
                     if (previousChallengeOwner != null) {
-                        try {
-                            previousChallengeOwner.getListener()
-                                    .onChallengeInterruptFinished(mSensorId);
-                        } catch (RemoteException e) {
-                            Slog.e(TAG, "Unable to notify interrupt finished", e);
+                        final ClientMonitorCallbackConverter listener =
+                                previousChallengeOwner.getListener();
+                        if (listener == null) {
+                            Slog.w(TAG, "Listener is null");
+                        } else {
+                            try {
+                                listener.onChallengeInterruptFinished(mSensorId);
+                            } catch (RemoteException e) {
+                                Slog.e(TAG, "Unable to notify interrupt finished", e);
+                            }
                         }
                     }
                 }