Docgen fails to export preconditions and/or precondition iterations from test plan or test executions

Description
Docgen fails to export preconditions and/or precondition iterations from test plan or test executions

How to reproduce

1. Create a TestPlan
2. Add a Test Execution with Tests, preconditions and/or and precondition iterations and create a TestRun
3. Try to export Test Run details starting the export from the Test Plan (it has been confirmed it's possible to reproduce this starting the export from Test Executions too)
   Example template used for testing: https://store.getxporter.app/view/138

Actual result/Attachments
DocGen errors out:

Expected result
Template exported without errors