Handle possible null PlaybackState

Fixes: 170706598
Test: atest SeekBarViewModelTest
Change-Id: I78428dd2d48658878957a7b58948a73d3f6bfaad
(cherry picked from commit f06e4ffffe)

packages/SystemUI/src/com/android/systemui/media/SeekBarViewModel.kt

@@ -91,9 +91,9 @@ class SeekBarViewModel @Inject constructor(@Background private val bgExecutor: R
         }
     private var playbackState: PlaybackState? = null
     private var callback = object : MediaController.Callback() {
-        override fun onPlaybackStateChanged(state: PlaybackState) {
+        override fun onPlaybackStateChanged(state: PlaybackState?) {
             playbackState = state
-            if (PlaybackState.STATE_NONE.equals(playbackState)) {
+            if (playbackState == null || PlaybackState.STATE_NONE.equals(playbackState)) {
                 clearController()
             } else {
                 checkIfPollingNeeded()

packages/SystemUI/tests/src/com/android/systemui/media/SeekBarViewModelTest.kt

@@ -654,4 +654,21 @@ public class SeekBarViewModelTest : SysuiTestCase() {
         fakeExecutor.runAllReady()
         verify(mockController).unregisterCallback(any())
     }
+
+    @Test
+    fun nullPlaybackStateUnregistersCallback() {
+        viewModel.updateController(mockController)
+        val captor = ArgumentCaptor.forClass(MediaController.Callback::class.java)
+        verify(mockController).registerCallback(captor.capture())
+        val callback = captor.value
+        // WHEN the callback receives a null state
+        callback.onPlaybackStateChanged(null)
+        with(fakeExecutor) {
+            advanceClockToNext()
+            runAllReady()
+        }
+        // THEN we unregister callback (as a result of clearing the controller)
+        fakeExecutor.runAllReady()
+        verify(mockController).unregisterCallback(any())
+    }
 }