Comment 7 for bug 1447256

I've confirmed Simos's observation. I've identified two variants of the problem, doing a test installation under VirtualBox:

1) The disk is unpartitioned (either with no partition table or with a GPT in which no partitions are defined) when ubiquity is launched. In this case, the error message appears AFTER the manual partitioning screen. Importantly, it does not appear to be possible to continue past this error in this case; both the "Continue" and "Go back" buttons are unresponsive. Thus, the claim in the release notes that this bug "is harmless since... you can proceed with the installation" is flat-out wrong.
2) The disk contains at least one partition when ubiquity is launched. (I tested with a single partition flagged as an ESP, but holding no filesystem.) In this case, the error appears BEFORE the manual partitioning screen, and the "Continue" button works. (I didn't test the "Go back" button.) In this case, the error is confusing but is not nearly so important.

Because there seems to be no way out in the first case except to terminate ubiquity (by rebooting or using command-line tools to kill the process), and because the only fix is the (non-obvious) workaround of creating a partition table and at least one partition, I consider this bug to be serious.