Fix NPE in InstantAppButtonsPreferenceController

- check for valid install options menu before trying to disable it.

Change-Id: Ibe644a506094e66b0f657092a25557f5096d4942
Fixes: 73767192
Test: make RunSettingsRoboTests

tests/robotests/src/com/android/settings/applications/appinfo/InstantAppButtonsPreferenceControllerTest.java

@@ -179,6 +179,15 @@ public class InstantAppButtonsPreferenceControllerTest {
         verify(menuItem, never()).setEnabled(false);
     }
 
+    @Test
+    public void onPrepareOptionsMenu_installMenuNotFound_shoulNotCrash() {
+        final Menu menu = mock(Menu.class);
+
+        mController.onPrepareOptionsMenu(menu);
+
+        // no crash
+    }
+
     @Test
     public void onOptionsItemSelected_shouldOpenAppStore() {
         final ResolveInfo resolveInfo = mock(ResolveInfo.class);