Make sure you have index.php in your main theme. Main theme must have index.php. I have same issue, and this was the case. Checked by:
$tema = wp_get_theme();
add_action('init', 'testate' );
If you like this question & answer and want to contribute, then write your question & answer and email to freewebmentor[@]gmail.com. Your question and answer will appear on FreeWebMentor.com and help other developers.