Dynamic grouping failed the first time

-Check if Worker is null
-Add test case

Bug: 154875482
Test: make -j50 RunSettingsRoboTests
Change-Id: I84572ef77cc9a48c3133d96fd2255754bc1cf67c

tests/robotests/src/com/android/settings/media/MediaOutputGroupSliceTest.java

@@ -156,6 +156,12 @@ public class MediaOutputGroupSliceTest {
         assertThat(rows).isEqualTo(3);
     }
 
+    @Test
+    public void getSlice_nullWorker_noException() {
+        sMediaDeviceUpdateWorker = null;
+        mMediaOutputGroupSlice.getSlice();
+    }
+
     @Test
     public void getSlice_withOneSelectableDevice_checkTitle() {
         mSelectableDevices.add(mDevice1);