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Many thanks for this! I was able to create bi-directional relationships using only the code. The relationship section however I have the following error being thrown:

Warning: Parameter 2 to RWMB_Post_Field::search_by_title() expected to be a reference, value given in C:\path\class-wp-hook.php on line 303
Call Stack
#  Time    Memory  Function    Location
1   0.0001  413488  {main}( )   ...\post.php:0
2   0.2734  13921328    require( 'C:\path\wp-admin\edit-form-advanced.php' ) ...\post.php:206
3   0.3613  14678792    do_meta_boxes( )    ...\edit-form-advanced.php:681
4   0.3681  14692504    RW_Meta_Box->show( ) ...\template.php:1395
5   0.3691  14694200    RWMB_Field::call( ) ...\meta-box.php:259
6   0.3691  14694640    RWMB_Object_Choice_Field::show( )   ...\field.php:580
7   0.3696  14695736    RWMB_Field::call( ) ...\object-choice.php:28
8   0.3696  14696176    RWMB_Post_Field::query( )   ...\field.php:580
9   0.3697  14699296    WP_Query->__construct( ) ...\post.php:155
10  0.3697  14699296    WP_Query->query( )   ...\class-wp-query.php:3576
11  0.3697  14699296    WP_Query->get_posts( )   ...\class-wp-query.php:3465
12  0.3699  14703168    apply_filters_ref_array( )  ...\class-wp-query.php:2131
13  0.3699  14703168    WP_Hook->apply_filters( )

Any ideas what could be causing this, please?

Kindly disregard this as I found a solution, I was missing the following fields:

'clone' => true,
 'sort_clone' => true,
 'collapsible' => true,
 'default_state' => 'collapsed',
 'group_title' => 'Question',