Usually when you skip a required field in the poll creation form, it says "This value is required." For the two radio button fields, however, it doesn't show such a message. Thus it is hard to figure out why the form submission failed.
This might actually be a bug with HTMLForm rather than SecureForm.
This seems to be more of a problem with HTMLForm; SecurePoll's include/pages/CreatePage.php uses that to create the form on poll creation page. The fields in question (Poll type, and Encryption) have a property 'required' => true and it is properly validated, yet HTMLForm fails to show a message next to the element when the validation fails. It only happens with radio buttons.
I can't think of other places were we generate radio buttons that are required, using the HTMLForm class. If one knows of an example, please share here. For now, I am also gonig to add HTMLForm tag.
Change 309920 had a related patch set uploaded (by Huji):
Display an error message when the validataion of a radio input fails because user did not provide any input.
Change 309920 merged by jenkins-bot:
Display an error message when the validataion of a radio input fails because user did not provide any input.