Running Portlet Causes 500 Internal server error

Document ID : KB000028467
Last Modified Date : 14/02/2018
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The Application Developer added a Portlet in the overview page and it gives the following error :
Error 500 - Internal Server Error. The server could not retrieve the document due to server-configuration or other technical problems. Contact your site administrator.

Running the NSQL query that is used in the portlet from query analyzer (in native query format) shows for one row that the dimension key column had the value as NULL.
Correct the NSQL query. The Dimension Key column value should be unique and cannot have NULL value. If not, portlets based on the query will not work as expected.