500 error on 5.0 upgrade caused by Genesis calls in mu-plugin

Hey all,
Posted this question in the WordPress forums and was told "those are all Genesis-specific function calls", go ask at Studiopress. So here I am! 🙂

I found a way around this, so this is more of a why did this happen/best practices question.

I experienced a 500 error during upgrade to WordPress 5.0. It affected only the backend/admin area; the front end was still accessible. This happened first in local dev environment/Valet, but repeated itself in other hosting environments. During troubleshooting, I found that disabling the following specific lines of widgets-init code in my site specific mu-plugin fixed the problem:

function tm_genesis_emc_widgets() {	
	genesis_register_sidebar( array(
	'id'          => 'get_quote',
	'name'        => __( 'EMC Get Quote area', 'genesischild' ),
	'description' => __( 'Adds HTML to last menu item', 'genesischild' ),
	) );

	genesis_register_sidebar( array(
	'id'          => 'woodgrain_social',
	'name'        => __( 'EMC Woodgrain Social area', 'genesischild' ),
	'description' => __( 'Adds HTML to bar under front page 1 area', 'genesischild' ),
	) );

	genesis_register_sidebar( array(
	'id'          => 'menu_page_sidebar',
	'name'        => __( 'Menu Page Floating Sidebar', 'genesischild' ),
	'description' => __( 'Adds an overlay sidebar area to Menu page', 'genesischild' ),
	) );
}
add_action( 'widgets_init', 'tm_genesis_emc_widgets' );

If I comment out this section of code, the upgrade runs as expected. If I uncomment this code after upgrade, everything continues to run as expected.

Can anyone help me understand why this caused a 500 error during upgrade, and what I should have done differently to prevent it and make my code more robust?

I have updated WordPress core many times before with this mu-plugin activated, but the 5.0 update was the first time this happened.

All plugins and themes are at latest versions. Disabling all other plugins (besides the mu-plugin) did not fix the problem. Genesis theme is at latest version.

Any thoughts? Thanks!