Document Generator throws NullPointerException when export a Test Plan

Short description:
After delete a pre condition issue associated to the Test, the Document Generator throws NullPointerException when export a Test Plan.

Steps to reproduce:
Install a Xray build from Marketplace.
On Document Generator, install the template "Test Plan Advanced with Cover Page (Xray 5)" from Store.
Create a test, and a pre-condition, then associate it to the test.
Associate test to test plan.
Execute a test run on test plan and select a status.
Delete the issue "Pre-Condition" that is associated to the test.
Go to test plan and try to export with document generator.

Actual Result:
The template doesn't export and an error message appears. In the log file appears: java.lang.NullPointerException

Expected Result:
The template should export as expected.