'f.get_entityLogicalName is not a function' error message when using addCustomView

Hi all!  I'm getting the following error message on a field after using the addCustomView call.

Error: "f.get_entityLogicalName is not a function"

If I run my FetchXML in the FetchXML Builder it works perfectly and get the returned results.  So I don't know what is causing the issue.  My code is as below:

function addCustomRMAProductView(executionContext)
{
    let formContext = executionContext.getFormContext();
    let viewID = '{00000000-0000-0000-0000-000000000001}';
    let entityName = 'msdyn_rmaproduct';
    let viewDisplayName = 'This is a custom view'; 
    let rmaNumberGUID = formContext.getAttribute('po_rma').getValue()[0].id;
    let rmaNumber = formContext.getAttribute('po_rma').getValue()[0].name;

    if (!rmaNumber)
    {
        console.log("RMA Number not Found.  Lookup field not populated");
    }
    else
    {  
        let fetchXML = "<fetch version=\"1.0\" output-format=\"xml-platform\" mapping=\"logical\" distinct=\"true\" aggregate=\"true\" >"
                + "<entity name=\"msdyn_rmaproduct\" >"
                    +"<attribute name=\"msdyn_productname\" aggregate=\"countcolumn\" alias=\"Count\" />"
                    + "<attribute name=\"msdyn_productname\" groupby=\"true\" alias=\"RMAProduct\" />"
                    + "<attribute name=\"msdyn_name\" groupby=\"true\" alias=\"name\" />"
                    + "<attribute name=\"statecode\" groupby=\"true\" alias=\"statecode\" />"
                    + "<attribute name=\"msdyn_rmaproductid\" groupby=\"true\" alias=\"msdyn_rmaproductid\" />"
                    + "<filter type=\"and\" >"
                        + "<condition attribute=\"po_customerasset\" operator=\"null\" />"
                        + "<condition attribute=\"msdyn_itemstatus\" operator=\"eq\" value=\"690970000\" />"
                    + "</filter>"
                    + "<link-entity name=\"msdyn_rma\" from=\"msdyn_rmaid\" to=\"msdyn_rma\" >"
                        + "<filter type=\"and\" >"
                            + "<condition attribute=\"msdyn_rmaid\" operator=\"eq\" uiname=\"" + rmaNumber + "\" uitype=\"msdyn_rma\" value=\"" + rmaNumberGUID + "\" />"
                        + "</filter>"
                    + "</link-entity>"
                + "</entity>"
            + "</fetch>";

        let layoutXML = 
            "<grid name=\"resultset\" object=\"10336\" jump=\"msdyn_name\" select=\"1\" icon=\"1\" preview=\"1\">"
                + "<row name=\"result\" id=\"msdyn_rmaproductid\">"
                    + "<cell name=\"RMAProduct\" width=\"100\" />"
                + "</row>"
            + "</grid>";

        formContext.getControl('po_customrmaproductfilterid').addCustomView(viewID, entityName, viewDisplayName, fetchXML, layoutXML, true)
    }
}

Any thoughts?