Check if order exists before adding order actions

[madeincosmos]

Fixes #652.

To reproduce:

1. Install AutomateWoo
2. Install Subscriptions
3. Install AutomateWoo -- Subscriptions Addon
4. Ensure PP Checkout is active
5. Create a new Workflow in AutomateWoo
   • Trigger: Subscription Order Paid
   • Subscription Order type: Parent
   • Action: Trigger Order Action
   • Order Action: Generate download permissions
6. Click "Save" to save the workflow.
7. With latest master, this triggers a fatal error:

2019-12-20T05:31:57+00:00 CRITICAL Uncaught Error: Call to a member function get_id() on bool in /nas/content/live/seventhrstage/wp-content/plugins/woocommerce-gateway-paypal-express-checkout/includes/class-wc-gateway-ppec-admin-handler.php:44
Stack trace:
#0 /nas/content/live/seventhrstage/wp-includes/class-wp-hook.php(288): WC_Gateway_PPEC_Admin_Handler->add_capture_charge_order_action(Array)
#1 /nas/content/live/seventhrstage/wp-includes/plugin.php(206): WP_Hook->apply_filters(Array, Array)
#2 /nas/content/live/seventhrstage/wp-content/plugins/automatewoo/includes/actions/order-trigger-action.php(31): apply_filters('woocommerce_ord...', Array)
#3 /nas/content/live/seventhrstage/wp-content/plugins/automatewoo/includes/abstracts/action.php(239): AutomateWoo\Action_Order_Trigger_Action->load_fields()
#4 /nas/content/live/seventhrstage/wp-content/plugins/automatewoo/includes/abstracts/action.php(222): AutomateWoo\Action->get_fields()
#5 /nas/content/live/seventhrstage/wp-content/plugins/automatewoo/includes/workflow.php(970): AutomateWoo\Action in /nas/content/live/seventhrstage/wp-content/plugins/woocommerce-gateway-paypal-express-checkout/includes/class-wc-gateway-ppec-admin-handler.php on line 44

Proposed fix resolves this error by checking if order exists for given post id.

[james-allan]

Thanks for the PR @madeincosmos. I've just made a slight coding change to remove an extra whitespace. Other than that this looks good to me.