nullpointerexception

our program is fully operational when upload to the server. but after a while, when we try to require data, ‘warning,wrong’ window is poping out, but data is still accessible.
we are using (java+dhtmlxconnector), data was uploaded by do method. “nullpointerException”is still poping out.
Please adivise what is the cause of this and how could I solve the problem.

Try to enable server side logs for connectors, and provide log report for the problematic scenario.