If you replace your reference to jquery.jqGrid.min.js with jquery.jqGrid.src.js you can get a better idea of the function that is causing the error, since that's the uncompressed version of the JQGrid code with meaningful variable and function names.

The error message you're getting looks the same as what I was getting before I made this replacement in an attempt to debug – see my post: /blog/?page_id=393/help/cannot-get-jqgrid-working-with-json-data/