When the user changes the report in the designer (for example creates a new one), the designer assigns a new report object.

Designer setup. Configure the designer, load the report template, process its callbacks, and connect the save action to the application.
var report = new Stimulsoft.Report.StiReport();
report.loadFile('Reports/SimpleList.mrt');

function App() {
    function onAssignedReport(args: Stimulsoft.Designer.AssignedReportArgs) {
        report = args.report;   // keep the latest report reference
    }

    var designerOptions = new Stimulsoft.Designer.StiDesignerOptions();
    designerOptions.appearance.fullScreenMode = true;

    return <Designer options={designerOptions} report={report} onAssignedReport={onAssignedReport} />;
}
onAssignedReport fires whenever the designer switches to a different report instance; storing args.report lets your application keep working with the exact template the user is editing (to preview, export or save it elsewhere).