NameError: global name 'worksheet' is not defined when using SheetProtection
*Created originally on Bitbucket by [lansman](https://bitbucket.org/%7B4a15d362-008c-4dff-abbc-c9e063860554%7D/)* ``` #!python wb = opx.Workbook(write_only=True) ws = wb.create_sheet("test") protection = opx.worksheet.protection.SheetProtection() protection.enable() ws.protection = protection wb.save("test.xlsx") ``` ``` #!python Traceback (most recent call last): File "C:/Users/user/.PyCharm2016.1/config/scratches/scratch_7.py", line 11, in <module> wb.save("test.xlsx") File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\workbook\workbook.py", line 261, in save save_dump(self, filename) File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\write_only.py", line 248, in save_dump writer.save(filename) File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\excel.py", line 222, in save self.write_data(archive, as_template=as_template) File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\excel.py", line 80, in write_data self._write_worksheets(archive) File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\excel.py", line 163, in _write_worksheets xml = sheet._write(self.workbook.shared_strings) File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\write_only.py", line 216, in _write self.close() File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\write_only.py", line 147, in close self.writer.close() File "C:\Python27\ArcGIS10.2\lib\site-packages\openpyxl\writer\write_only.py", line 122, in _write_header xf.write(worksheet.protection.to_tree()) NameError: global name 'worksheet' is not defined ``` openpyxl 2.3.5
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