PHP Warning: Invalid argument supplied for foreach() in

Так же само как и с foreach используйте альтернативный синтаксис if endif, получится должно в итоге вот так:

    <?php
    if (is_array($form_options['messages'])):
    $messages = $form_options['messages'];
    foreach ( $messages as $key => $message ):
        $elid = str_replace( '-', '_', $key );
        $content = apply_filters( 'wpmtst_l10n', $message['text'], wpmtst_get_l10n_context( 'form-messages' ), $key . ' : text' );
        ?>
        <tr>
            <th scope="row">
                <label for="<?php echo $elid; ?>">
                    <?php _ex( $message['description'], 'description', 'strong-testimonials' ); ?>
                </label>
                <input type="hidden" name="wpmtst_form_options[messages][<?php echo $key; ?>][description]"
                       value="<?php esc_attr_e( $message['description'] ); ?>"/>
            </th>
            <td>
                <?php if ( 'submission_success' == $elid ): ?>
                    <?php
                    $settings = array(
                        'textarea_name' => "wpmtst_form_options[messages][$key][text]",
                        'textarea_rows' => 10,
                    );
                    wp_editor( $content, $elid, $settings );
                    ?>
                <?php else: ?>
                    <input type="text" id="<?php echo $elid; ?>"
                           name="wpmtst_form_options[messages][<?php echo $key; ?>][text]"
                           value="<?php echo esc_attr( $content ); ?>" required />
                <?php endif; ?>
            </td>
            <td class="actions">
                <input type="button" class="button secondary restore-default-message"
                       value="<?php _ex( 'restore default', 'singular', 'strong-testimonials' ); ?>"
                       data-target-id="<?php echo $elid; ?>"/>
            </td>
        </tr>
    <?php 
      endforeach;
      endif; 
    ?>

Так же можете ознакомиться подробней с синтаксисом на официальном сайте PHP

Как это нет скобочек для foreach?

<?php
$messages = $form_options['messages'];
if ( is_array($messages) ) {
    foreach ( $messages as $key => $message ) {
        $elid = str_replace( '-', '_', $key );
        $content = apply_filters( 'wpmtst_l10n', $message['text'], wpmtst_get_l10n_context( 'form-messages' ), $key . ' : text' );
        ?>
        <tr>
            <th scope="row">
                <label for="<?php echo $elid; ?>">
                    <?php _ex( $message['description'], 'description', 'strong-testimonials' ); ?>
                </label>
                <input type="hidden" name="wpmtst_form_options[messages][<?php echo $key; ?>][description]"
                       value="<?php esc_attr_e( $message['description'] ); ?>"/>
            </th>
            <td>
                <?php if ( 'submission_success' == $elid ): ?>
                    <?php
                    $settings = array(
                        'textarea_name' => "wpmtst_form_options[messages][$key][text]",
                        'textarea_rows' => 10,
                    );
                    wp_editor( $content, $elid, $settings );
                    ?>
                <?php else: ?>
                    <input type="text" id="<?php echo $elid; ?>"
                           name="wpmtst_form_options[messages][<?php echo $key; ?>][text]"
                           value="<?php echo esc_attr( $content ); ?>" required />
                <?php endif; ?>
            </td>
            <td class="actions">
                <input type="button" class="button secondary restore-default-message"
                       value="<?php _ex( 'restore default', 'singular', 'strong-testimonials' ); ?>"
                       data-target-id="<?php echo $elid; ?>"/>
            </td>
        </tr>
    <?php
    }
}
?>