title: When Excel addin runs 'RunPython()', Workbook.caller() raise an error
body: <p>I found xlwings is a very exceptional project as a bridge between Excel and Python. So I am applying it to my excel addin development.</p>
  
  <p>But I got some problem. </p>
  
  <p>When excel addin calls python module, the Workbook.caller() does not return Workbook object(addin workbook itself)! Just Error!</p>
  
  <p>Instead of Workbook.caller(), I make use of 'Workbook() passing by 'Current ActiveWorkbook's name'. But in that case, I'm afraid that 'Optimize_connection= true' may raise memory garbage issue!</p>
  
  <p>(If 'IsAddin' property in VBA is off, then it runs very well)</p>
  
  <p>Is there anyone to help me?</p>
  
  <p>Thank you in advance.</p>