( ! ) Warning: Illegal string offset 'css' in /home/

Question

WordPress keeps giving me this error whenever I try to view my site. Any ideas on what the problem is?

The lines of code from line 233 till line 247:

protected function get_saved_assets_data() {
    $assets_data = get_option( self::ASSETS_DATA_KEY, [] );

    $content_type = $this->content_type;
    $assets_category = $this->assets_category;

    if ( ! isset( $assets_data[ $content_type ] ) ) {
        $assets_data[ $content_type ] = [];
    }

    if ( ! isset( $assets_data[ $content_type ][ $assets_category ] ) ) {
        $assets_data[ $content_type ][ $assets_category ] = [];
    }
    return $assets_data;
}