What is the test for determining whether an error in a jury verdict is harmless?

Under the Chapman standard, an error is harmless where the properly admitted evidence is overwhelming and the improperly admitted evidence is merely cumulative. (Harrington v. California (1969) 395 U.S. 250, 254.) "To find the error harmless we must find beyond a reasonable doubt that it did not contribute to the verdict, that it was unimportant in relation to everything else the jury considered on the issue in question. [Citation.]" (People v Song (2004) 124 Cal.App.4th 973, 984.)