Prevent IndexOutOfBoundException in VMenuBar.CustomMenuItem (#11285)

* Fixes #10814

* Add missing tests

Fixes #11374

client/src/main/java/com/vaadin/client/ui/VMenuBar.java

@@ -903,6 +903,7 @@ public class VMenuBar extends FocusableFlowPanel implements
             super.onLoad();
             if (getParentMenu() != null
                     && getParentMenu().getParentMenu() == null
+                    && getParentMenu().getItems().size() >= 1
                     && getParentMenu().getItems().get(0).equals(this)) {
                 getElement().setAttribute("tabindex", "0");
             } else {

uitest/src/main/java/com/vaadin/tests/components/menubar/MenuBarSmallWidth.java

@@ -0,0 +1,37 @@
+package com.vaadin.tests.components.menubar;
+
+import com.vaadin.annotations.Widgetset;
+import com.vaadin.server.ThemeResource;
+import com.vaadin.server.VaadinRequest;
+import com.vaadin.tests.components.AbstractTestUI;
+import com.vaadin.ui.Label;
+import com.vaadin.ui.MenuBar;
+
+@Widgetset("com.vaadin.DefaultWidgetSet")
+public class MenuBarSmallWidth extends AbstractTestUI {
+    @Override
+    protected void setup(VaadinRequest request) {
+        MenuBar barmenu = new MenuBar();
+        barmenu.setWidth("50px");
+        addComponent(barmenu);
+        // A top-level menu item that opens a submenu
+        MenuBar.MenuItem drinks = barmenu.addItem("Beverages", null, null);
+
+        // Submenu item with a sub-submenu
+        MenuBar.MenuItem hots = drinks.addItem("Hot", null, null);
+        hots.addItem("Tea", new ThemeResource("icons/tea-16px.png"), null);
+        hots.addItem("Coffee", new ThemeResource("icons/coffee-16px.png"),
+                null);
+
+        // Another submenu item with a sub-submenu
+        MenuBar.MenuItem colds = drinks.addItem("Cold", null, null);
+        colds.addItem("Milk", null, null);
+        colds.addItem("Weissbier", null, null);
+
+        // Another top-level item
+        MenuBar.MenuItem snacks = barmenu.addItem("Snacks", null, null);
+        snacks.addItem("Weisswurst", null, null);
+        snacks.addItem("Bratwurst", null, null);
+        snacks.addItem("Currywurst", null, null);
+    }
+}

uitest/src/test/java/com/vaadin/tests/components/menubar/MenuBarSmallWidthTest.java

@@ -0,0 +1,13 @@
+package com.vaadin.tests.components.menubar;
+
+import com.vaadin.tests.tb3.MultiBrowserTest;
+import org.junit.Test;
+
+public class MenuBarSmallWidthTest extends MultiBrowserTest {
+
+    @Test
+    public void noIndexOutOfBoundsExceptionPresent() {
+        openTestURL("debug");
+        assertNoErrorNotifications();
+    }
+}