Ignore component removal if the component has no parent (#4537)

Fixes #4528

flow-server/src/main/java/com/vaadin/flow/component/HasComponents.java

@@ -17,6 +17,8 @@
 
 import java.util.Objects;
 
+import org.slf4j.LoggerFactory;
+
 import com.vaadin.flow.dom.Element;
 
 /**
@@ -62,7 +64,13 @@ default void remove(Component... components) {
         for (Component component : components) {
             Objects.requireNonNull(component,
                     "Component to remove cannot be null");
-            if (getElement().equals(component.getElement().getParent())) {
+            Element parent = component.getElement().getParent();
+            if (parent == null) {
+                LoggerFactory.getLogger(HasComponents.class).debug(
+                        "Remove of a component with no parent does nothing.");
+                return;
+            }
+            if (getElement().equals(parent)) {
                 getElement().removeChild(component.getElement());
             } else {
                 throw new IllegalArgumentException("The given component ("

flow-server/src/test/java/com/vaadin/flow/component/HasComponentsTest.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2000-2018 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.component;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class HasComponentsTest {
+
+    @Tag("div")
+    private static class TestComponent extends Component
+            implements HasComponents {
+
+    }
+
+    @Test
+    public void remove_removeComponentWithNoParent() {
+        TestComponent component = createTestStructure();
+        TestComponent innerComponent = new TestComponent();
+
+        // No any exception is thrown
+        component.remove(innerComponent);
+    }
+
+    @Test
+    public void remove_removeComponentWithCorrectParent() {
+        TestComponent component = createTestStructure();
+        TestComponent innerComponent = new TestComponent();
+
+        long size = component.getChildren().count();
+
+        component.add(innerComponent);
+
+        component.remove(innerComponent);
+
+        Assert.assertEquals(size, component.getChildren().count());
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void remove_removeComponentWithDifferentParent() {
+        TestComponent component = createTestStructure();
+
+        TestComponent another = createTestStructure();
+        TestComponent innerComponent = new TestComponent();
+
+        another.add(innerComponent);
+
+        component.remove(innerComponent);
+    }
+
+    private TestComponent createTestStructure() {
+        TestComponent component = new TestComponent();
+        checkChildren(0, component);
+        component.add(new TestComponent(), new TestComponent(),
+                new TestComponent());
+        checkChildren(3, component);
+        return component;
+    }
+
+    private void checkChildren(int number, TestComponent component) {
+        Assert.assertEquals(number, component.getChildren().count());
+    }
+
+}

flow-server/src/test/java/com/vaadin/flow/component/UITest.java

@@ -307,13 +307,6 @@ public void removeComponent() {
         ComponentTest.assertChildren(ui);
     }
 
-    @Test(expected = IllegalArgumentException.class)
-    public void removeNotChildComponent() {
-        UI ui = new UI();
-        Text text = new Text("foo");
-        ui.remove(text);
-    }
-
     @Test
     public void setSession_attachEventIsFired()
             throws InvalidRouteConfigurationException {