Can't set `print_area` to None
Example: ``` from openpyxl import Workbook wb = Workbook() ws = wb.active # Get the current print area, correctly returns None print(ws.print_area) # Print area is set correctly ws.print_area = "A1:B10" print(ws.print_area) # Clear the print area ws.print_area = None # Error here "'NoneType' object is not iterable" print(ws.print_area) ``` Can't work out how to get a merge request in but you could fix this by changing line 892 of worksheet.py to: ``` if value != None: self._print_area = [absolute_coordinate(v) for v in value] else: self._print_area = None ``` Test (Line 420 of test_worksheet.py) ``` @pytest.mark.parametrize("cell_range, result", [ ("A1:F5", ["$A$1:$F$5"]), (["$A$1:$F$5"], ["$A$1:$F$5"],), (None, None), ] ```
issue