Cannot show grid with Filter Editor when a read-only form is present with a FileItem field

[jfarmer]

Please try the following code in the latest Feature Explorer in the latest Chrome version:

https://www.smartclient.com/smartcli...id=headerSpans

- The grid with showFilterEditor set to true is initially hidden. The form is visible and has canEdit set to true.
- Click the "Make Read-only" button to set canEdit to false on the form.
- Click the "Show Grid" button to show the grid.
- The grid does not show and a JavaScript error is logged to the Console:

Uncaught TypeError: Cannot read property 'tabIndexUpdatedCallback' of undefined

isc.ListGrid.create({
    ID: "countryListGrid", width:500, height: 150,  alternateRecordStyles:true,    dataSource: countryDS,
    fields: [
        { name: "countryName" }, { name: "continent" }, { name: "capital" }, { name: "countryCode" }
    ],
    showFilterEditor: true, filterOnKeypress: false, autoFetchData: true, visibility: "hidden"
});

isc.DynamicForm.create({
    ID: "testForm", width: 10, canEdit: true, canFocus: false, numCols: 2, colWidths: [ 5, 5 ],
    fields: [
        {    name: "logoFileButton",
            type: "file",
            accept: ".png",
            showFocused: false,
            title: "File"
        }   
    ]
});

isc.Button.create({
    ID: "disableFormButton", title: "Make Read-only",
    click: function() {
        testForm.setCanEdit(false);
    }
});

isc.Button.create({
    ID: "showGridButton", title: "Show Grid",
    click: function() {
        countryListGrid.show();
    }
});

isc.HLayout.create({
    ID: "buttonLayout", layoutMargin:5,  membersMargin: 10,
    members: [ disableFormButton, showGridButton ]
});

isc.VLayout.create({
    ID: "contentLayout",
    members: [ buttonLayout, countryListGrid, testForm ]
});

Browsers: Chrome 84 and IE 11
SmartClient versions: 11.1 and 12.1
(We use Chrome and SmartClient v11.1p_2020-05-30/PowerEdition in our production code.)