Meta Box

Hi, thanks for your investigation.
I dived into the Timeline Express and find out the lines doing the error. Actually, the function new_cmb2_box([...]) calls an error.

function new_cmb2_box( array $meta_box_config ) {
	return cmb2_get_metabox( $meta_box_config );
}

function cmb2_get_metabox( $meta_box, $object_id = 0, $object_type = '' ) {

	if ( $meta_box instanceof CMB2 ) {
		return $meta_box;
	}

	if ( is_string( $meta_box ) ) {
		$cmb = CMB2_Boxes::get( $meta_box );
	} else {
		// See if we already have an instance of this metabox
		$cmb = CMB2_Boxes::get( $meta_box['id'] );
		// If not, we'll initate a new metabox
		$cmb = $cmb ? $cmb : new CMB2( $meta_box, $object_id );
	}

	if ( $cmb && $object_id ) {
		$cmb->object_id( $object_id );
	}

	if ( $cmb && $object_type ) {
		$cmb->object_type( $object_type );
	}

	return $cmb;
}

There are used https://wordpress.org/plugins/cmb2/ plugin, that seems it is not compatible with your plugin. Any idea to fix that.