Blank Admin Screen on Update

Hello fellow wordpressers,
I am developing a theme for my own site, not for distribution, but as portfolio website for my blog and marker art. I enjoy doing the code, but I’m not very good at it, and I don’t really have any training. I learned it basically through googling tags and copy-pasting.

I made some changes to my functions.php file; added some image sizes and enabled post thumbnails. However, now when I make any update, the admin screen goes blank. It completes the update, but I have to hit the back button to get the screen to come back up. I can’t filter posts or do much of anything that requires more than one step. I’m kind of annoyed and I’m not sure what I did.

I found a similar problem posted by this guy: https://www.ads-software.com/support/topic/wp-admin-goes-blank-after-saving-changes-theme-issue?replies=8
However, I really don’t understand the answer.

Here is what my functions.php file looks like, can anyone tell me what might be causing this problem?

<?php
if ( function_exists('register_sidebar') )
    register_sidebars((2),array(
        'before_widget' => '<div id="%1$s" class="widget %2$s">',
    'after_widget' => '</div>',
            'before_title' => '<h3>',
        'after_title' => '</h3>',
    ));

if ( function_exists( 'add_image_size' ) ) {
	add_image_size( 'wideside', 250, 9999 );
	add_image_size( 'homepage-thumb', 9999, 250, true );
	add_image_size( 'display', 800, 9999, true );
}

?>

<?php add_theme_support( 'post-thumbnails' ); ?>