PHP Errors when ajax calls in checkout

Hello, a few others had already reported, that the action add_action( ‘posts_selection’, ‘mpwa_frontend_fields_init_posts_selection’ ); might be a problem. I also have issues and getting errors. Only if ajax calls are happening on the checkout – therefore if you do not run the function mpwa_frontend_fields_init_posts_selection if wp_doing_ajax() there is no more error.

Debug Info – with backtrace:

[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'Trying to get property 'ID' of non-object' at D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3998: D:\laragon\www\project\cms\wp-includes\query.php 584 calling is_page() | D:\laragon\www\project\content\plugins\woocommerce\includes\wc-conditional-functions.php 112 calling is_page() | D:\laragon\www\project\content\plugins\add-on-woocommerce-mailpoet\includes\class-mpwa-frontend-fields.php 214 calling is_checkout() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling mpwa_frontend_fields_init_posts_selection() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 2786 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3436 calling get_posts() | D:\laragon\www\project\cms\wp-includes\class-wp.php 620 calling query() | D:\laragon\www\project\cms\wp-includes\class-wp.php 737 calling query_posts() | D:\laragon\www\project\cms\wp-includes\functions.php 1274 calling main() | D:\laragon\www\project\cms\wp-blog-header.php 16 calling wp() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  Trying to get property 'ID' of non-object in D:\laragon\www\project\cms\wp-includes\class-wp-query.php on line 3998
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'Trying to get property 'post_title' of non-object' at D:\laragon\www\project\cms\wp-includes\class-wp-query.php 4000: D:\laragon\www\project\cms\wp-includes\query.php 584 calling is_page() | D:\laragon\www\project\content\plugins\woocommerce\includes\wc-conditional-functions.php 112 calling is_page() | D:\laragon\www\project\content\plugins\add-on-woocommerce-mailpoet\includes\class-mpwa-frontend-fields.php 214 calling is_checkout() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling mpwa_frontend_fields_init_posts_selection() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 2786 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3436 calling get_posts() | D:\laragon\www\project\cms\wp-includes\class-wp.php 620 calling query() | D:\laragon\www\project\cms\wp-includes\class-wp.php 737 calling query_posts() | D:\laragon\www\project\cms\wp-includes\functions.php 1274 calling main() | D:\laragon\www\project\cms\wp-blog-header.php 16 calling wp() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  Trying to get property 'post_title' of non-object in D:\laragon\www\project\cms\wp-includes\class-wp-query.php on line 4000
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'Trying to get property 'post_name' of non-object' at D:\laragon\www\project\cms\wp-includes\class-wp-query.php 4002: D:\laragon\www\project\cms\wp-includes\query.php 584 calling is_page() | D:\laragon\www\project\content\plugins\woocommerce\includes\wc-conditional-functions.php 112 calling is_page() | D:\laragon\www\project\content\plugins\add-on-woocommerce-mailpoet\includes\class-mpwa-frontend-fields.php 214 calling is_checkout() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling mpwa_frontend_fields_init_posts_selection() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 2786 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3436 calling get_posts() | D:\laragon\www\project\cms\wp-includes\class-wp.php 620 calling query() | D:\laragon\www\project\cms\wp-includes\class-wp.php 737 calling query_posts() | D:\laragon\www\project\cms\wp-includes\functions.php 1274 calling main() | D:\laragon\www\project\cms\wp-blog-header.php 16 calling wp() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  Trying to get property 'post_name' of non-object in D:\laragon\www\project\cms\wp-includes\class-wp-query.php on line 4002
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'wc_ajax_headers cannot set headers - headers already sent by  on line 0' at D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php 69: D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php 69 calling trigger_error() | D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php 87 calling wc_ajax_headers() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling do_wc_ajax() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\template-loader.php 13 calling do_action() | D:\laragon\www\project\cms\wp-blog-header.php 19 calling require_once() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  wc_ajax_headers cannot set headers - headers already sent by  on line 0 in D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php on line 69
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'Trying to get property 'ID' of non-object' at D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3998: D:\laragon\www\project\cms\wp-includes\query.php 584 calling is_page() | D:\laragon\www\project\content\plugins\woocommerce\includes\wc-conditional-functions.php 112 calling is_page() | D:\laragon\www\project\content\plugins\add-on-woocommerce-mailpoet\includes\class-mpwa-frontend-fields.php 214 calling is_checkout() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling mpwa_frontend_fields_init_posts_selection() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 2786 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3436 calling get_posts() | D:\laragon\www\project\cms\wp-includes\class-wp.php 620 calling query() | D:\laragon\www\project\cms\wp-includes\class-wp.php 737 calling query_posts() | D:\laragon\www\project\cms\wp-includes\functions.php 1274 calling main() | D:\laragon\www\project\cms\wp-blog-header.php 16 calling wp() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  Trying to get property 'ID' of non-object in D:\laragon\www\project\cms\wp-includes\class-wp-query.php on line 3998
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'Trying to get property 'post_title' of non-object' at D:\laragon\www\project\cms\wp-includes\class-wp-query.php 4000: D:\laragon\www\project\cms\wp-includes\query.php 584 calling is_page() | D:\laragon\www\project\content\plugins\woocommerce\includes\wc-conditional-functions.php 112 calling is_page() | D:\laragon\www\project\content\plugins\add-on-woocommerce-mailpoet\includes\class-mpwa-frontend-fields.php 214 calling is_checkout() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling mpwa_frontend_fields_init_posts_selection() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 2786 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3436 calling get_posts() | D:\laragon\www\project\cms\wp-includes\class-wp.php 620 calling query() | D:\laragon\www\project\cms\wp-includes\class-wp.php 737 calling query_posts() | D:\laragon\www\project\cms\wp-includes\functions.php 1274 calling main() | D:\laragon\www\project\cms\wp-blog-header.php 16 calling wp() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  Trying to get property 'post_title' of non-object in D:\laragon\www\project\cms\wp-includes\class-wp-query.php on line 4000
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'Trying to get property 'post_name' of non-object' at D:\laragon\www\project\cms\wp-includes\class-wp-query.php 4002: D:\laragon\www\project\cms\wp-includes\query.php 584 calling is_page() | D:\laragon\www\project\content\plugins\woocommerce\includes\wc-conditional-functions.php 112 calling is_page() | D:\laragon\www\project\content\plugins\add-on-woocommerce-mailpoet\includes\class-mpwa-frontend-fields.php 214 calling is_checkout() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling mpwa_frontend_fields_init_posts_selection() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 2786 calling do_action() | D:\laragon\www\project\cms\wp-includes\class-wp-query.php 3436 calling get_posts() | D:\laragon\www\project\cms\wp-includes\class-wp.php 620 calling query() | D:\laragon\www\project\cms\wp-includes\class-wp.php 737 calling query_posts() | D:\laragon\www\project\cms\wp-includes\functions.php 1274 calling main() | D:\laragon\www\project\cms\wp-blog-header.php 16 calling wp() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  Trying to get property 'post_name' of non-object in D:\laragon\www\project\cms\wp-includes\class-wp-query.php on line 4002
[27-Apr-2020 14:06:33 UTC] Backtrace from warning 'wc_ajax_headers cannot set headers - headers already sent by  on line 0' at D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php 69: D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php 69 calling trigger_error() | D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php 87 calling wc_ajax_headers() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 287 calling do_wc_ajax() | D:\laragon\www\project\cms\wp-includes\class-wp-hook.php 311 calling apply_filters() | D:\laragon\www\project\cms\wp-includes\plugin.php 478 calling do_action() | D:\laragon\www\project\cms\wp-includes\template-loader.php 13 calling do_action() | D:\laragon\www\project\cms\wp-blog-header.php 19 calling require_once() | D:\laragon\www\project\index.php 17 calling require()
[27-Apr-2020 14:06:33 UTC] PHP Notice:  wc_ajax_headers cannot set headers - headers already sent by  on line 0 in D:\laragon\www\project\content\plugins\woocommerce\includes\class-wc-ajax.php on line 69