Does testimony supporting a defense in a criminal case undermine the fundamental fairness of the criminal trial and thus precluding a possible lesser conviction for involuntary manslaughter?

testimony supporting a defense, and thus precluding a possible lesser conviction (here, for involuntary manslaughter), undermines the fundamental fairness of a criminal trial and should therefore be evaluated under the higher standard of Chapman v. California (1967) 386 U.S. 18. Under the Chapman standard, "before a[n] ... error can be held harmless, the court must be able to declare a belief that it was harmless beyond a reasonable doubt." (Id. at p. 24) We need not determine which standard applies here because, assuming any error took place, it was harmless under either standard.