How to open a new window from static method - JavaFX [duplicate]

I'm trying to create custom-popup via JavaFX, and have some trouble with initiating it from a static method.

How can I initiate a new window from a static method?

General information about my program - The user should type data and select/deselect a checkbox. Pressing on "submit" button runs a static method that does some stuff, and according to the user checkbox selection - run another method that does other stuff.

If the checkbox is deselected, I would like to open another window (custom popup).

However, I can't do that, since all my methods are static (can't change that). The method uploadCustomIndexWindow is defined as static, and therefore, when I try to initiate my custom-popup, I get the error

Cannot make a static reference to the non-static method getClass() from the type Object.

.

private static Index getStartEndIndex(String childFormat, boolean isFromExportTDP) {

    if(IndexMap.getIndexMap().get(childFormat) == null) {

        Index index;
        if (isFromExportTDP) {
            if(childFormat.equalsIgnoreCase("pdf")){
                index = new Index(childFormat, 2, 12);
            }
            else {
                index = new Index(childFormat, 2, 5);
            }
        }
        else{
            // Custom pop-up
            uploadCustomIndexWindow();

            index = new Index(childFormat, startIndex, endIndex);
        }
        IndexMap.getIndexMap().put(childFormat, index);
    }
    return IndexMap.getIndexMap().get(childFormat);
}



public static void uploadCustomIndexWindow() throws IOException{
    FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("CustomIndexScreen.fxml"));
    Parent root1 = (Parent) fxmlLoader.load();
    Stage stage = new Stage();
    stage.setScene(new Scene(root1));
    stage.setTitle("Custom Index Screen");
    stage.show();
}