Call save，Prompt list index out of range
#### Code Sample, a copy-pastable example if possible

import openpyxl
path = r'C:\Users\hcb\Desktop\test.xlsx'
wb = openpyxl.load_workbook(path)
wb.save('test.xlsx')

#### Problem description

Call save，Prompt list index out of range

#### Expected Output

Traceback (most recent call last):
  File "D:/python3/test1/myPackge/test2.py", line 4, in <module>
    wb.save('test.xlsx')
  File "C:\Python36\lib\site-packages\openpyxl\workbook\workbook.py", line 342, in save
    save_workbook(self, filename)
  File "C:\Python36\lib\site-packages\openpyxl\writer\excel.py", line 267, in save_workbook
    writer.save(filename)
  File "C:\Python36\lib\site-packages\openpyxl\writer\excel.py", line 248, in save
    self.write_data()
  File "C:\Python36\lib\site-packages\openpyxl\writer\excel.py", line 81, in write_data
    self._write_worksheets()
  File "C:\Python36\lib\site-packages\openpyxl\writer\excel.py", line 197, in _write_worksheets
    xml = ws._write()
  File "C:\Python36\lib\site-packages\openpyxl\worksheet\worksheet.py", line 870, in _write
    return write_worksheet(self)
  File "C:\Python36\lib\site-packages\openpyxl\writer\worksheet.py", line 99, in write_worksheet
    cols = ws.column_dimensions.to_tree()
  File "C:\Python36\lib\site-packages\openpyxl\worksheet\dimensions.py", line 230, in to_tree
    obj = col.to_tree()
  File "C:\Python36\lib\site-packages\openpyxl\worksheet\dimensions.py", line 182, in to_tree
    attrs = dict(self)
  File "C:\Python36\lib\site-packages\openpyxl\worksheet\dimensions.py", line 51, in __iter__
    value = getattr(self, key, None)
  File "C:\Python36\lib\site-packages\openpyxl\styles\styleable.py", line 95, in __get__
    return coll.names[idx]
IndexError: list index out of range


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