Can not create new order using Woocommerce RestApi

I create order by using Woocommerce RestApi. But after installing the WaNotifier plugin I am getting this below mentioned error and can't create order. But if I disable the plugin then I can create New order using RestApi.
 What to do?
Error ----
code: 'internal_server_error',
message: '

There has been a critical error on this website.

Learn more about troubleshooting WordPress.',
data: {
status: 500,
error: {
type: 1,
message: 'Uncaught Error: Call to a member function get() on null in /home1/adharsw/public_html/wp-content/plugins/notifier/includes/classes/integrations/class-notifier-woocommerce.php:1148\n' +
'Stack trace:\n' +
'#0 /home1/adharsw/public_html/wp-includes/class-wp-hook.php(326): Notifier_Woocommerce::notifier_save_session_on_new_order(3094)\n' +
'#1 /home1/adharsw/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array)\n' +
'#2 /home1/adharsw/public_html/wp-includes/plugin.php(517): WP_Hook->do_action(Array)\n' +
"#3 /home1/adharsw/public_html/wp-content/plugins/woocommerce/includes/data-stores/class-wc-order-data-store-cpt.php(103): do_action('woocommerce_new…', 3094, Object(WC_Order))\n" +
'#4 /home1/adharsw/public_html/wp-content/plugins/woocommerce/includes/class-wc-data-store.php(186): WC_Order_Data_Store_CPT->create(Object(WC_Order))\n' +
'#5 /home1/adharsw/public_html/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-order.php(214): WC_Data_Store->create(Object(WC_Order))\n' +
'#6 /home1/adharsw/public_html/wp-conten',
file: '/home1/adharsw/public_html/wp-content/plugins/notifier/includes/classes/integrations/class-notifier-woocommerce.php',
line: 1148
}
},
additional_errors: []