Fix empty TreeGrid (#9067)

client/src/main/java/com/vaadin/client/connectors/treegrid/TreeGridConnector.java

@@ -98,7 +98,8 @@ public class TreeGridConnector extends GridConnector {
             // Id of new hierarchy column. Choose first when nothing explicitly
             // set
             String newHierarchyColumnId = getState().hierarchyColumnId;
-            if (newHierarchyColumnId == null) {
+            if (newHierarchyColumnId == null
+                    && !getState().columnOrder.isEmpty()) {
                 newHierarchyColumnId = getState().columnOrder.get(0);
             }
 
@@ -106,6 +107,11 @@ public class TreeGridConnector extends GridConnector {
             Grid.Column<?, ?> newColumn = getColumn(newHierarchyColumnId);
             Grid.Column<?, ?> oldColumn = getColumn(oldHierarchyColumnId);
 
+            if (newColumn == null && oldColumn == null) {
+                // No hierarchy column defined
+                return;
+            }
+
             // Unwrap renderer of old column
             if (oldColumn != null
                     && oldColumn.getRenderer() instanceof HierarchyRenderer) {

server/src/main/java/com/vaadin/data/provider/HierarchicalDataCommunicator.java

@@ -140,6 +140,8 @@ public class HierarchicalDataCommunicator<T> extends DataCommunicator<T> {
             sendData(0, dataObjects);
             getActiveDataHandler().addActiveData(items.stream());
             getActiveDataHandler().cleanUp(items.stream());
+        } else {
+            getClientRpc().reset(0);
         }
 
         setPushRows(Range.withLength(0, 0));

uitest/src/main/java/com/vaadin/tests/components/treegrid/TreeGridEmpty.java

@@ -0,0 +1,14 @@
+package com.vaadin.tests.components.treegrid;
+
+import com.vaadin.server.VaadinRequest;
+import com.vaadin.tests.components.AbstractTestUI;
+import com.vaadin.ui.TreeGrid;
+
+public class TreeGridEmpty extends AbstractTestUI {
+
+    @Override
+    protected void setup(VaadinRequest request) {
+        TreeGrid<String> grid = new TreeGrid<>();
+        addComponent(grid);
+    }
+}

uitest/src/test/java/com/vaadin/tests/components/treegrid/TreeGridEmptyTest.java

@@ -0,0 +1,19 @@
+package com.vaadin.tests.components.treegrid;
+
+import static org.junit.Assert.assertFalse;
+
+import org.junit.Test;
+import org.openqa.selenium.By;
+
+import com.vaadin.tests.tb3.SingleBrowserTest;
+
+public class TreeGridEmptyTest extends SingleBrowserTest {
+
+    @Test
+    public void empty_treegrid_initialized_correctly() {
+        setDebug(true);
+        openTestURL();
+        assertNoErrorNotifications();
+        assertFalse(isElementPresent(By.className("v-errorindicator")));
+    }
+}