​Problems with displaying widgets in WP 5.8​

On updated themes fixed the problem by selecting at Theme options->General->Widget Classic Editor and enabling this feature
# for themes that have not been updated, add this code on the main theme or theme-child functions.php file to adjust the display of the Widget

add_filter( 'use_widgets_block_editor', '__return_false' );

add_filter( 'gutenberg_use_widgets_block_editor', '__return_false');