@Contract("null -> null")
  public static @Nullable PsiMethod getFunctionalInterfaceMethod(PsiClass aClass) {
    final MethodSignature methodSignature = getFunction(aClass);
    return methodSignature != null ? getMethod(aClass, methodSignature) : null;
  }
@Contract("null -> null")
  public static @Nullable PsiMethod getFunctionalInterfaceMethod(PsiClass aClass)