[Python] How to resolve the error: "cannot initialize variable with None; optionals are...

I'm using TakeProfit's Indie Script Indicator. I'm getting an error when trying to initialize a variable to None.

Here's a simplified version of the code:

# indie:lang_version = 5
from math import nan, isnan
from indie import indicator, param, plot, color, MutSeriesF
from indie.algorithms import Atr

@indicator('SuperTrend', overlay_main_pane=True)
@param.float('atr_mult', default=1.0, min=0.1, max=10.0, title='ATR Multiplier')
@param.int('n_atr', default=4, min=1, title='ATR Length')
@param.str('ma_type', default='HULL', options=['SMA', 'EMA', 'WMA', 'RMA', 'VWMA', 'HULL'], title='MA Type')
@param.bool('paint_bars', default=True, title='Color Bars')
@plot.line(title='SuperTrend')
@plot.bar_color()
def Main(self, atr_mult, n_atr, ma_type, paint_bars):
hl2 = (self.high[0] + self.low[0]) / 2
atr = Atr.new(n_atr, ma_type)

up = MutSeriesF.new(hl2 + atr_mult * atr[0])
dn = MutSeriesF.new(hl2 - atr_mult * atr[0])
st = MutSeriesF.new(nan)

if isnan(st[1]):
st[0] = up[0]
else:
if self.close[0] < st[1]:
st[0] = up[0]
else:
st[0] = dn[0]

bar_col = None
if paint_bars:
bar_col = color.RED if self.close[0] < st[0] else color.GREEN

return st[0], bar_col



But Indie compiler gives this error:

Error: cannot initialize bar_col with None; in Indie language optionals are explicit, so to make this assignment, you need to declare bar_col as indie.Optional[...]
​

How should I properly initialize a variable that can be either a color or None in TakeProfit's Indie Script language?

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