Fix an NPE caused by the previous fix to Grid's frozen columns (#11444)

* Fix an NPE caused by the previous fix to Grid's frozen columns (#10653)

If the user managed to get two frozen column count changes into a single server round-trip before the component had been attached, the previous fix  #11346 lead to a null pointer exception since there was no diff state available.

This fix adds a null check before removing the frozen column count value from the diff state so that this will not happen.

Closes #10653

3 files changed, 92 insertions, 2 deletions

diff --git a/server/src/main/java/com/vaadin/ui/Grid.java b/server/src/main/java/com/vaadin/ui/Grid.java
index 650bf7cab5..9ee57512b2 100644
--- a/server/src/main/java/com/vaadin/ui/Grid.java
+++ b/server/src/main/java/com/vaadin/ui/Grid.java
@@ -3232,8 +3232,12 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
             final String diffStateKey = "frozenColumnCount";
             UI ui = getUI();
             if (ui != null) {
-                ui.getConnectorTracker().getDiffState(Grid.this)
-                        .remove(diffStateKey);
+                JsonObject diffState = ui.getConnectorTracker()
+                        .getDiffState(Grid.this);
+                // if diffState is not present, there's nothing for us to clean
+                if (diffState != null) {
+                    diffState.remove(diffStateKey);
+                }
             }
         }
         getState().frozenColumnCount = numberOfColumns;
diff --git a/uitest/src/main/java/com/vaadin/tests/components/grid/GridFrozenColumnNPE.java b/uitest/src/main/java/com/vaadin/tests/components/grid/GridFrozenColumnNPE.java
new file mode 100644
index 0000000000..1d7f5901f3
--- /dev/null
+++ b/uitest/src/main/java/com/vaadin/tests/components/grid/GridFrozenColumnNPE.java
@@ -0,0 +1,60 @@
+package com.vaadin.tests.components.grid;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import com.vaadin.server.VaadinRequest;
+import com.vaadin.tests.data.bean.Person;
+import com.vaadin.ui.Button;
+import com.vaadin.ui.Grid;
+import com.vaadin.ui.renderers.NumberRenderer;
+
+public class GridFrozenColumnNPE extends SimpleGridUI {
+
+    private Grid<Person> grid;
+
+    @Override
+    protected void setup(VaadinRequest request) {
+        grid = new Grid<Person>();
+        grid.setSizeFull();
+        init();
+        getLayout().addComponent(grid);
+
+        grid.setFrozenColumnCount(1);
+
+        Button button = new Button("change frozen count");
+        button.addClickListener(event -> {
+            reInit();
+        });
+        getLayout().addComponent(button);
+    }
+
+    @Override
+    protected List<Person> createPersons() {
+        List<Person> persons = new ArrayList<>();
+        for (int i = 0; i < 10; ++i) {
+            Person person = new Person();
+            person.setFirstName("First " + i);
+            person.setLastName("Last" + i);
+            person.setAge(i);
+            persons.add(person);
+        }
+        return persons;
+    }
+
+    protected void init() {
+        grid.addColumn(Person::getFirstName);
+        grid.addColumn(Person::getLastName);
+        grid.addColumn(Person::getAge, new NumberRenderer());
+
+        grid.setItems(createPersons());
+        grid.setFrozenColumnCount(1);
+        grid.setFrozenColumnCount(2);
+    }
+
+    protected void reInit() {
+        grid.removeAllColumns();
+        init();
+    }
+
+}
diff --git a/uitest/src/test/java/com/vaadin/tests/components/grid/GridFrozenColumnNPETest.java b/uitest/src/test/java/com/vaadin/tests/components/grid/GridFrozenColumnNPETest.java
new file mode 100644
index 0000000000..29b91d3326
--- /dev/null
+++ b/uitest/src/test/java/com/vaadin/tests/components/grid/GridFrozenColumnNPETest.java
@@ -0,0 +1,26 @@
+package com.vaadin.tests.components.grid;
+
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+import com.vaadin.testbench.elements.ButtonElement;
+import com.vaadin.testbench.elements.GridElement;
+import com.vaadin.tests.tb3.MultiBrowserTest;
+
+public class GridFrozenColumnNPETest extends MultiBrowserTest {
+
+    @Test
+    public void testFrozenColumnNPE() {
+        openTestURL();
+        GridElement grid = $(GridElement.class).first();
+
+        assertTrue(grid.getCell(0, 0).isFrozen());
+
+        ButtonElement button = $(ButtonElement.class).first();
+        button.click();
+
+        assertTrue(grid.getCell(0, 1).isFrozen());
+    }
+
+}