text: Title: Exception error when exporting to HTML
  Body: Hello, 
  
  I tried exporting my map to HTML, the code is same in the documentation.
  `embed_minimal_html('export.html', views=[fig])`
  It works fine on my computer, but my friend has encountered the following exception error.
  <img width="982" alt="error" src="https://user-images.githubusercontent.com/34975332/54467860-d38d6f80-47c2-11e9-80ae-b05dc33c9a67.png">
  
  I am afraid we forget to do something before running the code.
  What can I do to fix this?
  
  Thank you for your help!
  Best, 
  Iris