instance is empty in function widget()

Your widget really has a lot of issues and completely breaks my layout on my back-end widget page. I suggest that you start at the Widget API to learn the proper way how to code a widget

Here is a couple of issues I have picked up:

• Reputable sources should not be suggesting or even use extract(). All traces of this function was completely removed from core, except one instance (as far as I can understand from the trac ticket) that has not yet been solved. This should say a lot about the function

• Your widget form is the culprit for breaking my layout. I have not even tried to look at that as there are more than two issues here. I have just completely changed that and also used get_categories() as I previously suggested to get the dropdown working and stop my layout from breaking

• Your widget function that should output the front end info is missing a lot of elements. I’m not sure about the exact layout you are after or what your plan is on usage, so I have just done the following: (This is the bare necessities that you should have)

  ?><section id="<?php echo $sectionID; ?>"><?php
  
  echo $args['before_widget'];
  if ( $title ) {
      echo $args['before_title'] . $title . $args['after_title'];
  }               
  echo "SOMETHING IS MISSING HERE";
  
  echo $args['after_widget']; 
  
  ?></section><?php
  

• Class names should have the words starting with camelcase according the naming convention section in the coding standards

I have also added a cache inside the widget to speed it up

Here is a working example of how your widget should look like. I have tested it, it works on my side. I also get output on the front end with the code given, you will just need to modify this and add the missing parts where I stated echo "SOMETHING IS MISSING HERE";

/**
 * Alliance_Post_Builder widget class
 *
 * @since 1.0.0
*/
class Alliance_Post_Builder extends WP_Widget {

    public function __construct() {
    parent::__construct(
        'alliance_post_builder', // Base ID
        __( 'Alliance post builder', 'alliance' ), // Name
        array( 'description' => __( 'A post builder', 'alliance' ), ) // Args
    );
        $this->alt_option_name="widget_alliance_posts";

        add_action( 'save_post', array($this, 'flush_widget_cache') );
        add_action( 'deleted_post', array($this, 'flush_widget_cache') );
        add_action( 'switch_theme', array($this, 'flush_widget_cache') );
    }

    public function widget( $args, $instance ) {
        $cache = array();
        if ( ! $this->is_preview() ) {
            $cache = wp_cache_get( 'widget_alliance_build_posts', 'widget' );
        }

        if ( ! is_array( $cache ) ) {
            $cache = array();
        }

        if ( ! isset( $args['widget_id'] ) ) {
            $args['widget_id'] = $this->id;
        }

        if ( isset( $cache[ $args['widget_id'] ] ) ) {
            echo $cache[ $args['widget_id'] ];
            return;
        }

        ob_start();

        $title          = ( ! empty( $instance['title'] ) ) ? $instance['title'] : __( 'A new section', 'alliance' );
        /** This filter is documented in wp-includes/default-widgets.php */
        $title          = apply_filters( 'widget_title', $title, $instance, $this->id_base );
        $sectionID      = ( ! empty( $instance['sectionID'] ) ) ? $instance['sectionID'] : __( 'A new section', 'alliance' );
        $category       = $instance['category'];  


        ?><section id="<?php echo $sectionID; ?>"><?php

        echo $args['before_widget'];
        if ( $title ) {
            echo $args['before_title'] . $title . $args['after_title'];
        }               
        echo "SOMETHING IS MISSING HERE";

        echo $args['after_widget']; 

        ?></section><?php

        if ( ! $this->is_preview() ) {
            $cache[ $args['widget_id'] ] = ob_get_flush();
            wp_cache_set( 'widget_alliance_build_posts', $cache, 'widget' );
        } else {
            ob_end_flush();
        }
    }

    public function update( $new_instance, $old_instance ) {
        $instance                   = $old_instance;
        $instance['title']          = strip_tags( $new_instance['title'] );
        $instance['sectionID']      = strip_tags( $new_instance['sectionID'] );
        $instance['category']       = $new_instance['category'];
        $this->flush_widget_cache();

        $alloptions = wp_cache_get( 'alloptions', 'options' );
        if ( isset($alloptions['widget_alliance_posts']) )
            delete_option('widget_alliance_posts');

        return $instance;
    }

    public function flush_widget_cache() {
        wp_cache_delete('widget_alliance_build_posts', 'widget');
    }

    public function form( $instance ) {

        $title      = isset( $instance['title'] ) ? esc_attr( $instance['title'] ) : '';
        $sectionID  = isset( $instance['sectionID'] ) ? esc_attr( $instance['sectionID'] ) : '';
        $category   = isset( $instance['category'] ) ? $instance['category'] : -1;
        ?>

        <p>
            <label for="<?php echo $this->get_field_id( 'sectionID' ); ?>"><?php _e( 'Section Id:' ); ?></label> 
            <input class="widefat" id="<?php echo $this->get_field_id( 'sectionID' ); ?>" name="<?php echo $this->get_field_name( 'sectionID' ); ?>" type="text" value="<?php echo esc_attr( $sectionID ); ?>">
        </p>

        <p>
            <label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo $title; ?>" />
        </p>

        <p>
            <label for="<?php echo $this->get_field_id('category'); ?>"><?php _e( 'Category Name:' )?></label>
            <select id="<?php echo $this->get_field_id('category'); ?>" name="<?php echo $this->get_field_name('category'); ?>">
                <?php 
                $this->categories = get_categories();
                foreach ( $this->categories as $cat ) {
                    $selected = ( $cat->term_id == esc_attr( $category ) ) ? ' selected = "selected" ' : '';
                    $option = '<option '.$selected .'value="' . $cat->term_id;
                    $option = $option .'">';
                    $option = $option .$cat->name;
                    $option = $option .'</option>';
                    echo $option;
                }
                ?>
            </select>
        </p>

    <?php
    }

}

add_action( 'widgets_init', function () {
    register_widget( 'Alliance_Post_Builder' );
});