Blank/White screen when opening some Service Desk requests from the customer portal

Issue Summary

If a Service Desk ticket contains mentions of other tickets in public comments, a customer won't be able to open this ticket if this user does not have access to the mentioned tickets, and will see a blank page.

The issue can also occur when hyperlinks to other sites are mentioned in either the ticket summary or ticket description

Steps to reproduce

• Install Service Desk 4.4.2
• Create a new Service Desk project
• Create a customer account in this project (let's call the user customer1)
• Create 2 tickets in the Service Desk project:
  • 1 ticket that customer1 has access to, by being the reporter or a participant (let's call this ticket BASICSD-2)
  • 1 ticket that customer1 does not have access to (let's call this ticket BASICSD-3)
• Log into Jira as an agent
• Open BASICSD-2, add a new public comment and include the internal URL of the issue BASICSD-3 (<JIRA_BASE_URL>/browse/BASICSD-3) in the comment
  
• Log into the customer portal as the user customer1
• Go to My Requests, and click on the request BASICSD-2

Expected results

The request BASICSD-2 should be opened in the customer portal.

Actual results

• The request BASICSD-2 can't be opened and the following happens:
  • the ticket shows as a blank screen and a 500 error is thrown in the UI
    
  • the following error is thrown in the Jira logs:

    2019-10-18 10:03:12,746 http-nio-8080-exec-10 ERROR xxxxxx XXXxXXXXxX ubczia XXX.XX.XX.XX /servicedesk/customer/portal/1/ABC-123 [o.a.c.c.C.[.[localhost]..[default]] Servlet.service() for servlet [default] threw exception
    java.lang.NullPointerException
            at java.util.regex.Matcher.appendReplacement(Matcher.java:804)
            at com.atlassian.servicedesk.internal.feature.customer.request.UrlReplacer.replace(UrlReplacer.java:41)
            at com.atlassian.servicedesk.internal.feature.customer.request.IssueUrlConverterImpl.replaceIssueUrlsWithPortalRequestUrls(IssueUrlConverterImpl.java:71)
            at com.atlassian.servicedesk.internal.feature.customer.request.CustomerTextRendererImpl.updateCustomerTextIntertal(CustomerTextRendererImpl.java:165)
            at com.atlassian.servicedesk.internal.feature.customer.request.CustomerTextRendererImpl.updateTextForCustomer(CustomerTextRendererImpl.java:150)
            at com.atlassian.servicedesk.internal.feature.customer.request.CustomerTextRendererImpl.renderComment(CustomerTextRendererImpl.java:84)
            at com.atlassian.servicedesk.internal.feature.customer.request.activitystream.responses.ActivityResponseManagerImpl.buildCommentResponse(ActivityResponseManagerImpl.java:96)
            at com.atlassian.servicedesk.internal.feature.customer.request.activitystream.responses.ActivityResponseManagerImpl.getResponse(ActivityResponseManagerImpl.java:72)
            at 
    

• In some situations, requests on the portal won't be shown at all:
  

Workaround

2 possible workarounds:

1. Remove the URL <JIRA_BASE_URL>/browse/BASICSD-3) from the comment added to BASICSD-2
2. Add the impacted user (customer1) as a participant of the request BASICSD-3