JIRA Throws NullPointerException When Invalid Sprint Is Inserted In Create or Edit Issue Screen

Steps to reproduce:

1. Install GreenHopper 6 into JIRA 5.1.1
2. add Sprint field to the Create/Edit screen
3. create sprint in the GreenHopper/Agile(eg: Sprint 1)
4. create new issue in JIRA while inserting "Sprint 1" to the field Sprint or edit existing issue and insert "Sprint 1" into the Sprint field. Value will not be added, JIRA does not throws error message in the UI but, will throws error in the log:

    2012-08-17 18:24:58,287 http-8483-5 ERROR      [500ErrorPage.jsp] Exception caught in 500 page For input string: "sprint 1"
   java.lang.NumberFormatException: For input string: "sprint 1"
   	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
   	at java.lang.Long.parseLong(Long.java:410)
   	at java.lang.Long.valueOf(Long.java:525)
   	at com.atlassian.greenhopper.customfield.sprint.SprintCFType.getSingularObjectFromString(SprintCFType.java:78)
   	at com.atlassian.greenhopper.customfield.sprint.SprintCFType.getValueFromCustomFieldParams(SprintCFType.java:153)
   	at com.atlassian.greenhopper.customfield.sprint.SprintCFType.getValueFromCustomFieldParams(SprintCFType.java:37)
   	at com.atlassian.jira.issue.fields.CustomFieldImpl.updateIssue(CustomFieldImpl.java:1123)
   

5. for the same issue TEST-1, insert non-existing Sprint number(eg:5). JIRA throws error message in the UI as shown in the screenshot SprintError.png . In the logs, error message:

   2012-08-17 18:07:29,168 http-8483-2 ERROR admin 1087x305x1 wsqoqb 127.0.0.1 /secure/QuickEditIssue.jspa [jira.bc.issue.DefaultIssueService] Exception occurred editing issue: java.lang.NullPointerException
   java.lang.NullPointerException
   	at com.atlassian.greenhopper.customfield.sprint.SprintCFType.convertTypeToDbValue(SprintCFType.java:244)
   	at com.atlassian.greenhopper.customfield.sprint.SprintCFType.convertTypeToDbValue(SprintCFType.java:37)
   

6. try to create a new issue and try to insert invalid Sprint number(eg:3). Error message thrown in the JIRA UI as per Sprint-CreateIssue.png

JIRA should handle the invalid value gracefully instead of throwing NullPointerException error or not responding.