Refer to https://tracker.moodle.org/browse/MDL-83920

Steps to reproduce:

• Create a new empty course
• Go to the course as a teacher > More > Question bank
• Create a default question bank
• Go to that the newly created question bank
• Select "Export" from the dropdown
• Select any format > Export questions to file
• It giving the error message saying "No questions were found that could be exported. Make sure that you have selected a category to export that contains questions."

But it would be more wise if downloading an empty question category was signalized beforehand.