NullPointerException in loading data due to orphaned sub-task items issue

Summary

For some specific scenarios (orphaned sub-task items) Jira Portfolio blocks the update calls for affected items:

• "Delete" (or any other like "Move" or "Convert") item ends up with an error:

java.lang.NullPointerException
	at com.atlassian.rm.teams.customfields.team.IssueTeamService.getTeamForNonSubTask(IssueTeamService.java:50) [?:?]
	at com.atlassian.rm.teams.customfields.team.IssueTeamService.getTeam(IssueTeamService.java:35) [?:?]
	at com.atlassian.rm.teams.customfields.team.TeamCustomFieldTypeHelper.getTeam(TeamCustomFieldTypeHelper.java:117) [?:?]

At the same time the JIRA lexorank engine is handling that issue with a warning only:

NodeReindexServiceThread:thread-1 WARN [greenhopper.customfield.sprint.SprintHistoryEntryFactory] Unable to fetch parent issue for RMBRITS-317

Environment

• JIRA Software: 7.2.10
• Portfolio for JIRA: 2.6.1

Steps to Reproduce

There's no easy step to reproduce. The orphaned sub-task item is required (that may be related to unidentified JIRA bug).

Expected Results

"Delete" action should be successful (even with a warning).

Actual Results

Any UI update is failing with NPE for the specific item.

Workaround

• This issue could not be reproduced locally. 
• The customers could execute the Jira database integrity checker.
• Focus on the first section named "Check Issue Relations".
  If it finds any errors, choose to fix them. You can refer to the article Sub-tasks are not Correctly Associated with Parent Issues in Jira
• Then try to run the re-indexing and see if the issue is resolved.

Quick fix : 

This issue could be solved by running the "integrity checker".

Trouble shooting : 

https://confluence.atlassian.com/jirakb/troubleshoot-a-reindex-failure-in-jira-server-429917142.html