WordPress5.0でプラグインの新規追加画面・編集画面が真っ白になった場合の対処 ref: https://qiita.com/webbingstudio@github/items/f86e964226e7617a72f4

acf_on_wp5.php

function WS_remove_block_editor( $user_block_editor, $post ) {
    if ( $post->post_type === 'acf-field-group' ) {
        $use_block_editor = false;
    } else {
        $use_block_editor = true;
    }
    return $use_block_editor;
}
add_filter( 'use_block_editor_for_post', 'WS_remove_block_editor', 10, 2 );

mwwpf_on_wp5.php

function WS_remove_block_editor( $user_block_editor, $post ) {
    if (
        ( $post->post_type === 'acf-field-group' )
        ||( $post->post_type === 'mw-wp-form' )
    ) {
        $use_block_editor = false;
    } else {
        $use_block_editor = true;
    }
    return $use_block_editor;
}
add_filter( 'use_block_editor_for_post', 'WS_remove_block_editor', 10, 2 );