Fix NavigationBarController NPE

SystemUI would crash if NavigationBarFragment wasn't available
yet.

Test: mp sysuig # boots
Test: atest NavigationBarControllerTest
Fixes: 141939419
Change-Id: I1e099c5cc3c23c05140d550dd870722f57055013

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

@@ -237,6 +237,7 @@ public class NavigationBarController implements Callbacks {
 
     /** @return {@link AssistHandleViewController} (only on the default display). */
     public AssistHandleViewController getAssistHandlerViewController() {
-        return getDefaultNavigationBarFragment().getAssistHandlerViewController();
+        NavigationBarFragment navBar = getDefaultNavigationBarFragment();
+        return navBar == null ? null : navBar.getAssistHandlerViewController();
     }
 }

packages/SystemUI/tests/src/com/android/systemui/statusbar/NavigationBarControllerTest.java

@@ -28,6 +28,7 @@ import static org.mockito.Mockito.doNothing;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.reset;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -240,4 +241,10 @@ public class NavigationBarControllerTest extends SysuiTestCase {
 
         verify(mSecondaryNavBar).disableAnimationsDuringHide(eq(500L));
     }
+
+    @Test
+    public void testGetAssistHandlerViewController_noCrash() {
+        reset(mNavigationBarController.mNavigationBars);
+        mNavigationBarController.getAssistHandlerViewController();
+    }
 }