fix: add null check in setFooterToolbar

Close #131
Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/gridhelpers/FooterToolbarGridHelper.java

@@ -24,6 +24,7 @@
 import com.vaadin.flow.component.grid.FooterRow.FooterCell;
 import com.vaadin.flow.component.grid.Grid;
 import java.io.Serializable;
+import java.util.Objects;
 import lombok.RequiredArgsConstructor;
 
 @SuppressWarnings("serial")
@@ -35,6 +36,7 @@ class FooterToolbarGridHelper implements Serializable {
   private FooterCell footerCell;
 
   public void setFooterToolbar(Component toolBar) {
+    Objects.requireNonNull(toolBar, "Toolbar component must not be null");
     Grid<?> grid = helper.getGrid();
     if (grid.getColumns().isEmpty()) {
       throw new IllegalStateException("Cannot set footer toolbar: Grid columns have not been configured.");

src/test/java/com/flowingcode/vaadin/addons/gridhelpers/test/FooterToolbarTest.java

@@ -19,9 +19,11 @@
  */
 package com.flowingcode.vaadin.addons.gridhelpers.test;
 
+import com.flowingcode.vaadin.addons.gridhelpers.GridHelper;
 import com.flowingcode.vaadin.addons.gridhelpers.GridHelper;
 import com.vaadin.flow.component.grid.Grid;
 import com.vaadin.flow.component.orderedlayout.HorizontalLayout;
+import org.junit.Assert;
 import org.junit.Test;
 
 public class FooterToolbarTest {
@@ -42,4 +44,16 @@ public void testSetFooterToolbarBeforeColumnsConfiguredThrowsException() {
     var toolbarFooter = new HorizontalLayout();
     GridHelper.addToolbarFooter(grid, toolbarFooter);
   }
+
+  @Test(expected = NullPointerException.class)
+  public void testSetFooterToolbarWithNullToolbarThrowsException() {
+    Grid<Bean> grid = new Grid<>(Bean.class, false);
+    grid.addColumn(x -> x).setHeader("Header");
+    try {
+      GridHelper.addToolbarFooter(grid, null);
+    } catch (NullPointerException e) {
+      Assert.assertEquals("Toolbar component must not be null", e.getMessage());
+      throw e;
+    }
+  }
 }