hopefully fix #4272 -- "export_to_ipynb → Cannot read property 'get_ipynb' of undefined"
hopefully fix #4272 -- "export_to_ipynb → Cannot read property 'get_ipynb' of undefined"

DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc