JQGrid Disable Add Button in SubGrid Based on Status Column in SubGrid

I have a sub grid that has a status column defined. I would like to disable the Add button on its navGrid until subgrid's status column's value is 'Completed'.

Here is a snippet of the HTML that is being generated that I'm (unsuccessfuly) trying to manipulate: (I think I need to use the first and last elements)...???

<div id="USAttorneyFoldersGrid_43" class="tablediv">
<div id="gbox_USAttorneyFoldersGrid_43_t" class="ui-jqgrid ui-widget ui-widget-content ui-corner-all" dir="ltr" style="width: 826px;">
<div id="lui_USAttorneyFoldersGrid_43_t" class="ui-widget-overlay jqgrid-overlay"></div>
<div id="load_USAttorneyFoldersGrid_43_t" class="loading ui-state-default ui-state-active" style="display: none;">Loading...</div>
<div id="editmodUSAttorneyFoldersGrid_43_t" class="ui-widget ui-widget-content ui-corner-all ui-jqdialog jqmID1" dir="ltr" style="width: 300px; height: auto; z-index: 950; overflow: hidden; top: 294px; left: 136px; display: none;" tabindex="-1" role="dialog" aria-labelledby="edithdUSAttorneyFoldersGrid_43_t" aria-hidden="true">
<div id="gview_USAttorneyFoldersGrid_43_t" class="ui-jqgrid-view" style="width: 826px;">
<div id="rs_mUSAttorneyFoldersGrid_43_t" class="ui-jqgrid-resize-mark">&nbsp;</div>
<div id="p_USAttorneyFoldersGrid_43_t" class="scroll ui-state-default ui-jqgrid-pager ui-corner-bottom" style="width: 826px;" dir="ltr">
<div id="pg_p_USAttorneyFoldersGrid_43_t" class="ui-pager开发者_运维知识库-control" role="group">

This is my selector that doesn't appear to work:

$('#USAttorneyFoldersGrid_43').navGrid('pg_p_USAttorneyFoldersGrid_43_t', { add: false });

What am I missing here?

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You posted almost no code, but I hope that my old answer with the demo gives you information which you need.