text: Title: Simple bug on downloading data using example Backtesting with vectorbt
  Body: Just tried to go step by step with the example [https://github.com/twopirllc/pandas-ta/blob/main/examples/VectorBT_Backtest_with_Pandas_TA.ipynb](url)
  
  It didn't work, received and error: UnboundLocalError: local variable 'df' referenced before assignment