A test plan cannot be linked to the parent test plan.

[SowmiyaKaruna]

Description of problem

A new test plan cannot be linked to any of the existing test plans. Basically, nesting is not working.

Version or commit hash (if applicable)

13.5-Enterprise

Steps to Reproduce

1. Navigate to Testing --> New Test Plan --> Select Product and Enter Test Plan name --> Give the Parent Test Plan ID that we want to link --> Press the save button.
2. Once the test plan has been created, navigate to Edit and look for the parent test plan.
3. Look into the test plan structure.

Actual results

After Step1: Test plan created with no errors shown.

After Step2: The parent test plan is empty when we look at the created test plan in edit mode.

After Step3: The nested plan structure missing the created test plan.

Although it is being added to the product, it is not linked to the parent test plan that we have given.

Expected results

Parent test plan should be able to be linked to the new test plans.

Additional info (Python traceback, logs, etc.)

[atodorov]

Looks like a duplicate of #3765.

See #3765 (comment) for more info.