TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'

[mbeacom]

https://sentry.gitcoin.co/gitcoin/gitcoin-prod-backend/issues/577/

TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'
(1 additional frame(s) were not displayed)
...
  File "retail/views.py", line 407, in results
    context = build_stat_results(keyword)
  File "retail/utils.py", line 239, in build_stat_results
    results = build_stat_results_helper(keyword)
  File "retail/utils.py", line 360, in build_stat_results_helper
    total_tips_usd = sum([tip.value_in_usdt for tip in Tip.objects.filter(network='mainnet').exclude(txid='')])

TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'