Use async overload.

modules/basic-theme/src/Volo.Abp.AspNetCore.Mvc.UI.Theme.Basic/Themes/Basic/Layouts/Application.cshtml

@@ -58,7 +58,7 @@
         @(await Component.InvokeAsync<PageAlertsViewComponent>())
         <div id="AbpContentToolbar">
             <div class="text-end mb-2">
-                @RenderSection("content_toolbar", false)
+                @await RenderSectionAsync("content_toolbar", false)
             </div>
         </div>
         @await Component.InvokeLayoutHookAsync(LayoutHooks.PageContent.First, StandardLayouts.Application)