In a large skillet, bring the olive oil over medium-high heat and add the diced chicken. Stir until coated with oil. Cook, stirring occasionally, for 5-7 minutes until chicken pieces are cooked through. Drain if necessary and stir in taco seasoning to coat. Remove to a container and allow to cool. Place in the refrigerator until chilled. When chicken is completely cooled, transfer it to a mixing bowl and add the corn, black beans, tomatoes and radishes. Stir lightly until combined. Mash the avocado with a fork and then transfer to a small dish. Stir in the sour cream until combined. Spoon the avocado mixture into the bowl with the chicken mixture and then stir together until the chicken salad is coated in the avocado mixture. Stir in cilantro or scallions if using. Serve chilled or at room temperature.