Handle missing migration source volume.

Users can try migrating primary storage while the current location
is missing/unmounted.  Fail gracefully instead of runtime restarting.

Bug: 21927076
Change-Id: I40645f8ccea05154e7cbacd188f6cba5f4dbbdc4
diff --git a/packages/SystemUI/src/com/android/systemui/usb/StorageNotification.java b/packages/SystemUI/src/com/android/systemui/usb/StorageNotification.java
index 92cfaa1..180d918 100644
--- a/packages/SystemUI/src/com/android/systemui/usb/StorageNotification.java
+++ b/packages/SystemUI/src/com/android/systemui/usb/StorageNotification.java
@@ -499,8 +499,10 @@
         final PendingIntent intent;
         if (privateVol != null && privateVol.getDisk() != null) {
             intent = buildWizardReadyPendingIntent(privateVol.getDisk());
-        } else {
+        } else if (privateVol != null) {
             intent = buildVolumeSettingsPendingIntent(privateVol);
+        } else {
+            intent = null;
         }
 
         final Notification notif = new Notification.Builder(mContext)