PR: https://github.com/ManageIQ/manageiq-ui-classic/pull/6380
Verified in CFME 5.11.1.1.20191122174937_707df01 by creating and deleting multiple generic objects and observing that 404 flash error occurred. The issue in the additional info: ("The custom button is deleted from the UI but if you try to add a custom button of the same name as the one just deleted you hit a "Server Error" and cannot add it. If you click "Add" again it results in a "Name already taken" flash message. If you then hit "Cancel", the button is still created!") also appears to have been fixed.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2019:4201