Hi Ravi,
Chart_1.getSelectedMember() will return a member and you are trying to concatenate it with a string.
Please use the following syntax,
APPLICATION.openNewWindow("http://<server>:8080/BOE/OpenDocument/opendoc/openDocument.jsp?sIDType=CUID&iDocID=AQNHxxx" + "&lsSPlant=" + CHART_1.getSelectedMember("0PLANT").internalKey + "&sRefresh=Y");
Regards,
Swapnil Koti