input: Write a function to find the first non-repeating character in a given list. If such a character exists, return it; otherwise, return None. If the list is empty, return None.
  assert first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1])==None
  assert first_non_repeating_char([1, 2, 3, 4, 1, 4, 2])==3
  assert first_non_repeating_char([5, 6, 5, 6, 7])==7
  assert first_non_repeating_char([8, 9, 8, 9, 10])==10
  assert first_non_repeating_char([])==None
output: def first_non_repeating_char(lst):
      if not lst:
          return None
      count = {}
      for item in lst:
          if item in count:
              count[item] += 1
          else:
              count[item] = 1
      for item in lst:
          if count[item] == 1:
              return item
      return None