Add check if global $current_screen is set

Hi,

Inside the method add_checkbox_auto_protect_file there should be an extra check if $screen is set.

The method is checking if get_current_screen() exists but in case that screen.php is included elsewhere this check is not enough.

Unfortunately this has led to a conflict with another plugin causing the following error: “Trying to get property ‘id’ of non-object”

Is there anywhere I could open up a pull request?

Adding the check isset($screen) inside add_checkbox_auto_protect_file would solve the problem:

if (isset($screen) && 'media' === $screen->id && 'add' == $screen->action) {
// 
}