Check for Shelf icon Nullness.

It's possible for it to be null, from other checks in the code.

Bug: None
Test: Build
Change-Id: I2b9c46690b608a97f6e323c3060641d1fda13c5d

packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationIconAreaController.java

@@ -349,6 +349,9 @@ public class NotificationIconAreaController implements
     }
 
     private void updateShelfIcons() {
+        if (mShelfIcons == null) {
+            return;
+        }
         updateIconsForLayout(entry -> entry.getIcons().getShelfIcon(), mShelfIcons,
                 true /* showAmbient */,
                 true /* showLowPriority */,