Editor : Select2 error when value exists d.getBaseType is not a function

Hi folks - thanks in advance for any help.

I am using editor

I can initialise a select2 select and fill values from the server without issue, using the code for the column.

However when that column has a value and I click to inline edit I see this error in the console, I cannot inline edit any column in the row now :
~~~~
I cannot suss out what i am doing wrong and any help appreciated !

ScriptResource.axd?d=NJmAwtEo3Ipnlaxl6CMhvqH1eehdFFnfCSnQ0epjjv0ykHECQfRcC8SKfXlfRHuc5BjXSyZD8RAmuj769cAFXfQLSKp4qnwe5mVCR78Cjig4QrSH6GKmwzBQ-5dDPHxm6xbxDj0oclnTAJ5TxwDc_MOF8UiQeLbPnCu3hbslEWE1&t=49337fe8:5 Uncaught TypeError: d.getBaseType is not a function
at Sys._getBaseMethod (ScriptResource.axd?d=NJmAwtEo3Ipnlaxl6CMhvqH1eehdFFnfCSnQ0epjjv0ykHECQfRcC8SKfXlfRHuc5BjXSyZD8RAmuj769cAFXfQLSKp4qnwe5mVCR78Cjig4QrSH6GKmwzBQ-5dDPHxm6xbxDj0oclnTAJ5TxwDc_MOF8UiQeLbPnCu3hbslEWE1&t=49337fe8:5:11475)
at Type.callBaseMethod (ScriptResource.axd?d=NJmAwtEo3Ipnlaxl6CMhvqH1eehdFFnfCSnQ0epjjv0ykHECQfRcC8SKfXlfRHuc5BjXSyZD8RAmuj769cAFXfQLSKp4qnwe5mVCR78Cjig4QrSH6GKmwzBQ-5dDPHxm6xbxDj0oclnTAJ5TxwDc_MOF8UiQeLbPnCu3hbslEWE1&t=49337fe8:5:7313)
at e (jquery.min.js:4:13154)
at Ab (jquery.min.js:4:13039)
at n.param (jquery.min.js:4:13400)
at Function.ajax (jquery.min.js:4:9596)
at Editor.set (editor.select2.js:221:15)
at Field._typeFn (dataTables.editor.js:6595:23)
at Field.set (dataTables.editor.js:6446:18)
at Field._multiValueCheck (dataTables.editor.js:6570:22)`

        {
                            label: "MDT Type:",
                            name: "MDTType",
                            className: '',
                            opts: {
                                ajax: {
                                    url: "/Webservices/DDL.ashx",
                                    dataType: 'json',
                                    delay: 250,
                                    data: function (params) {
                                    
                                        return {
                                            searchterm: params.term, // search term
                                            DDL: 'mdt'
                                         
                                        };
                                    },
                                    processResults: function (data, params) {
        
                                        return {
        
        
                                            results: data
        
                                        };
                                    },
                                    cache: true
                                },
                                optionsPair:{ label: 'text', value: 'id' }
                            },
                            type: "select2"
                        }