A Critical Update is being activated in Summer ’19: any URL Parameters utilized in Lightning need to have “namespace” prefix, otherwise they are dropped…
https://releasenotes.docs.salesforce.com/en-us/spring19/release-notes/rn_forcecom_general_namespace_prefix_cruc_reminder.htm
This affects the URL_CreateRecordCmp because the parameters being utilized by it are stripped by Lightning now.
To see this in action: in a sandbox that has Summer '19 (or the Critical Update activated)... this link will not work:
/lightning/cmp/c__URL_CreateRecordCmp?objectName=Case
the screen will refresh and the URL will drop all of the parameters… resulting in a blank screen. (the code errors calling action "c.getFieldDescribeMap" for null parameters)
if you use this link:
/lightning/cmp/c__URL_CreateRecordCmp?c__objectName=Case
you will still get the blank screen after refresh, but notice that the objectName parameter will carry through to the new page. (Still getting a blank screen because the parameter is now starting with c__ which is not expected…)
I'm going to adjust the code (and my URL creation code throughout) to prefix the parameters with "c__"
(Sorry, I wish I was better with git, I'd upload my changes once I make them... but I'm not really familiar with how it works to pull/push/etc... I'll relate my changes below once I make them.)