title: AttributeError: 'NoneType' object has no attribute 'read'
body: <p>I'm trying to save an Excel file using openpyxl and after executing my code I had this Error.</p>
  
  <p>My code:</p>
  
  <pre><code>def assemble_files(self):
      global names
  
      wb = load_workbook(names[0])
      sheet = wb.get_active_sheet()                           
      exportFile = asksaveasfilename( defaultextension=".xlsx", filetypes=(("Excel file", "*.xlsx"),("All Files", "*.*") ))
      wb.save(exportFile)
      messagebox.showinfo("Information", "Files successfully assembled")`
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Exception in Tkinter callback
  Traceback (most recent call last):
    File "C:\Users\mohamed.louati\AppData\Local\Programs\Python\Python36\lib\tkinter\__init__.py", line 1699, in __call__
      return self.func(*args)
    File "C:\Users\mohamed.louati\Desktop\aines\ID_Extractor.py", line 98, in assemble_files
      wb.save(exportFile)
    File "C:\Users\mohamed.louati\AppData\Local\Programs\Python\Python36\lib\site-packages\openpyxl\writer\excel.py", line 180, in _write_comment
      vml = fromstring(self.workbook.vba_archive.read(ws.legacy_drawing))
  AttributeError: 'NoneType' object has no attribute 'read'
  </code></pre>
  
  <p>can any one hep in solving this problem</p>