Fixes NullPointerException introduced in ag/9382746

Test: tested locally
BUG:142332706
Change-Id: Iba99a0346f5481671fe284004ceb8772e31b570e
FIX:142332706

packages/SystemUI/src/com/android/systemui/assist/AssistModule.java

@@ -21,6 +21,7 @@ import android.os.Handler;
 import android.os.HandlerThread;
 import android.os.SystemClock;
 
+import androidx.annotation.Nullable;
 import androidx.slice.Clock;
 
 import com.android.internal.app.AssistUtils;
@@ -68,6 +69,7 @@ public abstract class AssistModule {
     }
 
     @Provides
+    @Nullable
     static AssistHandleViewController provideAssistHandleViewController(
             NavigationBarController navigationBarController) {
         return navigationBarController.getAssistHandlerViewController();

packages/SystemUI/src/com/android/systemui/statusbar/NavigationBarController.java

@@ -233,11 +233,13 @@ public class NavigationBarController implements Callbacks {
     }
 
     /** @return {@link NavigationBarFragment} on the default display. */
+    @Nullable
     public NavigationBarFragment getDefaultNavigationBarFragment() {
         return mNavigationBars.get(DEFAULT_DISPLAY);
     }
 
     /** @return {@link AssistHandleViewController} (only on the default display). */
+    @Nullable
     public AssistHandleViewController getAssistHandlerViewController() {
         NavigationBarFragment navBar = getDefaultNavigationBarFragment();
         return navBar == null ? null : navBar.getAssistHandlerViewController();