8256283: IndexOutOfBoundsException when sorting a TreeTableView

Reviewed-by: kcr

modules/javafx.controls/src/main/java/javafx/scene/control/TreeTableView.java

@@ -579,7 +579,7 @@ public static int getNodeLevel(TreeItem<?> node) {
         @Override public Boolean call(TreeTableView table) {
             try {
                 TreeItem rootItem = table.getRoot();
-                if (rootItem == null) return false;
+                if (rootItem == null || rootItem.getChildren().isEmpty()) return false;
 
                 TreeSortMode sortMode = table.getSortMode();
                 if (sortMode == null) return false;

modules/javafx.controls/src/test/java/test/javafx/scene/control/TreeTableViewTest.java

@@ -753,6 +753,32 @@ private void verifySelectionAfterPermutation() {
         treeTableView.sort();
     }
 
+    @Test public void testNoIOOBEWhenSortingAfterSelectAndClearRootChildren() {
+        TreeTableView<String> ttv = new TreeTableView<>();
+        TreeItem<String> root = new TreeItem<>("root");
+        TreeItem<String> child = new TreeItem<>("child");
+        root.getChildren().add(child);
+        root.setExpanded(true);
+        ttv.setRoot(root);
+        ttv.setShowRoot(false);
+
+        TreeTableColumn<String, String> ttc = new TreeTableColumn<>("Column");
+        ttv.getSortOrder().add(ttc);
+
+        ttv.getSelectionModel().select(0);
+        root.getChildren().remove(0);
+        ControlTestUtils.runWithExceptionHandler(() -> {
+            ttv.sort();
+        });
+    }
+
+    @Test public void testNPEWhenRootItemIsNull() {
+        TreeTableView<String> ttv = new TreeTableView<>();
+        ControlTestUtils.runWithExceptionHandler(() -> {
+            ttv.sort();
+        });
+    }
+
     @Test public void testChangingSortPolicyUpdatesItemsList() {
         TreeTableColumn<String, String> col = initSortTestStructure();
         col.setSortType(DESCENDING);