What is the test for reversal of a verdict where the jury failed to agree on a unanimous verdict?

Relevant to defendants' challenge, when "the jury is 'misinstructed on an element of the offense . . . reversal . . . is required unless we are able to conclude that the error was harmless beyond a reasonable doubt.' [Citations.]" (People v. Wilkins (2013) 56 Cal.4th 333, 348.) However, "[t]he failure to give a unanimity instruction may be harmless error if we can conclude beyond a reasonable doubt that all jurors must have unanimously agreed on the act(s) constituting the offense." (People v. Norman (2007) 157 Cal.App.4th 460, 466.)