Add null check for worker

Add null check to avoid crash when worker cannot get through uri.

Bug: 128492874
Test: make -j42 RunSettingsRoboTests
Change-Id: I789ce0bfa98c0b0b145a8605c58c88a77eeb76a9

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

@@ -93,6 +93,13 @@ public class MediaOutputSliceTest {
         mMediaOutputSlice.init(TEST_PACKAGE_NAME, mMediaDeviceUpdateWorker);
     }
 
+    @Test
+    public void getSlice_workerIsNull_shouldNotCrash() {
+        mMediaOutputSlice.init(TEST_PACKAGE_NAME, null);
+
+        mMediaOutputSlice.getSlice();
+    }
+
     @Test
     public void getSlice_shouldHaveActiveDeviceName() {
         mDevices.clear();