Problem in using Customizer

Hey Only Change the code like this 

function cult_customizer_register( $wp_customize ) {
    $wp_customize->add_section( 'cult_topic_thumb' , array(
    'title'       => __( 'Topic of the Week' ),
    'priority'    => 36,
    'description' => 'Allows you to upload an image for your topic of the week section.',
) );
    $wp_customize->add_setting( 'display_topic_thumb' );
    $wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'display_topic_thumb', array(
    'label'    => __( 'Display Topic of the Week Thumb' ),
    'section'  => 'cult_topic_thumb',
    'settings' => 'display_topic_thumb',
) ) );
}
add_action('customize_register', 'cult_customizer_register');

And in your index.php call diplay_topic_thumb 

<div class="topic_week">
            <h1 class="section_heading"> TOPIC OF THE WEEK </h1>
            <?php if ( get_theme_mod( 'display_topic_thumb' ) ) : ?>
            <div class="photo">
                <a href="https://wordpress.stackexchange.com/questions/172426/<?php echo esc_url( home_url("https://wordpress.stackexchange.com/" ) ); ?>' rel="home"><img src="<?php echo esc_url( get_theme_mod( "display_topic_thumb' ) ); ?>' alt="<?php echo esc_attr( get_bloginfo( "name', 'display' ) ); ?>'></a>
            </div>
            <?php else : ?>
            <?php endif; ?>
        </div>

It will work fine for you

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