input: Write a python function to find the value with the highest frequency in a dictionary.
  assert max_freq_val({'a': 1, 'b': 2, 'c': 2, 'd': 3}) == 2
  assert max_freq_val({'x': 10, 'y': 10, 'z': 20, 'w': 20, 'v': 20}) == 20
  assert max_freq_val({'p': 5, 'q': 5, 'r': 5, 's': 4}) == 5
output: def max_freq_val(d):
      from collections import Counter
      if not d:
          return None
      value_counts = Counter(d.values())
      max_freq_value = max(value_counts, key=value_counts.get)
      return max_freq_value