Import of question in lesson module produces PHP warnings

When trying to import a hot potato question file, I get the following PHP warnings:

( ! ) Notice: Undefined property: qformat_hotpot::$course in D:\VertrigoServ\www\moodle_19\question\format\hotpot\format.php on line 30
Call Stack

1. Time Memory Function Location
   1 0.0348 110464 {main}( ) ..(null):0
   2 1.4263 18412608 qformat_default->importprocess( ) ..(null):68
   3 1.4292 18587648 qformat_hotpot->readquestions( ) ..(null):42
   
   ( ! ) Notice: Trying to get property of non-object in D:\VertrigoServ\www\moodle_19\question\format\hotpot\format.php on line 30
   Call Stack
   # Time Memory Function Location
   1 0.0348 110464 {main}

   ( ) ..(null):0
   2 1.4263 18412608 qformat_default->importprocess( ) ..(null):68
   3 1.4292 18587648 qformat_hotpot->readquestions( ) ..(null):42