Adding another check if you launch calendar from command line.

calendar/resources/content/calendarImportExport.js

@@ -67,7 +67,7 @@ const extensionCsv      = ".csv";
 const filterRdf         = "iCalendar RDF";
 const extensionRdf      = ".rdf";
 
-if( opener.gICalLib )
+if( opener && opener.gICalLib )
    gICalLib = opener.gICalLib;
 
 // convert to and from Unicode for file i/o

calendar/resources/content/importExport.js

@@ -67,7 +67,7 @@ const extensionCsv      = ".csv";
 const filterRdf         = "iCalendar RDF";
 const extensionRdf      = ".rdf";
 
-if( opener.gICalLib )
+if( opener && opener.gICalLib )
    gICalLib = opener.gICalLib;
 
 // convert to and from Unicode for file i/o