$instance variable checking null difficulty

Hello, I am currently developing a widget plugin and I am getting an input from user and that input is stored in $instance['code']

I would like to use this as argument to restrict the number of posts shown on the main website. However, if the input is not provided, then I want to set the default as 10 (so display 10 posts).

It worked fine when I used form as I use isset function to determine whether the input was null but now, in widget function that does not work and I want to know how can I achieve this.

Here is part of my code (I tested that the variable behaves correctly!)

public function widget($args, $instance) 

	{

		extract($args, EXTR_SKIP);

		//global WP theme-driven "before widget" code
		echo $before_widget;

		// code before your user input
		//echo '<iframe width="560" height="315" src="//www.youtube.com/embed/';

		//echo $instance['code'];

		// code after your user input
		//echo '"frameborder="0" allowfullscreen></iframe>';

		if($instance['code'] == null){
			$numPosts = 10;
		}else{
			$numPosts = $instance['code'];
		}

		$cateName = $instance['title'];

		$category_id = get_cat_ID($cateName);
		$catquery = new WP_Query( 'cat=' .$category_id);

		if($catquery->have_posts()):
			while($numPosts > 0 && $catquery->have_posts()) : $catquery->the_post();
			$numPosts -= 1;?>
			<article class="post">
			<h2> <a href="<?php the_permalink(); ?>"> <?php the_title();?></a> </h2>
			<?php the_post_thumbnail(array(20,20)); ?>
			written by <?php  the_author(); ?>
			<h3 id="comments"> <?php comments_number('no responses', 'one response', '% responses'); ?> </h3>
			<?php the_content(); ?>
			</article>
			<?php endwhile;

		else :
			echo '<p> No content found </p>';

		endif; 

		wp_reset_query();

		f//global WP theme-driven "after widget" code
		echo $after_widget;
	}

if($instance['code'] == null){
			$numPosts = 10;
		}else{
			$numPosts = $instance['code'];
		}

This part is the intention but does not work. Can anyone help?