…on_menu()

If you use `_{module}_civix_insert_navigation_menu()` to insert menu items, you might sometimes get a warning like this:

```
Error message
Notice: Undefined index: child in _foobar_civix_insert_navigation_menu() (line 296 of /Users/totten/buildkit/build/dmaster/sites/all/modules/civicrm/ext/org.civicrm.foobar/foobar.civix.php).
```