Null-check selected alarm before setting ringtone.

Bug: 24291458
Change-Id: I0ee8b95cd41e3284900db062560aace918fa1b30
diff --git a/src/com/android/deskclock/AlarmClockFragment.java b/src/com/android/deskclock/AlarmClockFragment.java
index 220f137..d443ba5 100644
--- a/src/com/android/deskclock/AlarmClockFragment.java
+++ b/src/com/android/deskclock/AlarmClockFragment.java
@@ -238,6 +238,10 @@
 
                 // Set the ringtone uri on the alarm.
                 final Alarm alarm = mAlarmTimeClickHandler.getSelectedAlarm();
+                if (alarm == null) {
+                    LogUtils.e("Could not get selected alarm to set ringtone");
+                    return;
+                }
                 alarm.alert = uri;
 
                 // Save the change to alarm.