Fix `TaskModel#updateFiltered` to not add tasks as new

src/main/java/cz/muni/fi/pv168/project/model/db/TaskModel.java

@@ -5,6 +5,7 @@ import cz.muni.fi.pv168.project.db.DataAccessException;
 import cz.muni.fi.pv168.project.db.interfaces.DataAccessObject;
 import cz.muni.fi.pv168.project.ui.dialog.error.ErrorDialog;
 
+import java.util.ArrayList;
 import java.util.function.Predicate;
 
 public class TaskModel extends UpdatableDataModel<Task> {
@@ -16,11 +17,13 @@ public class TaskModel extends UpdatableDataModel<Task> {
     public void updateFiltered(Predicate<Task> predicate) {
         clear();
         try {
+            var tasks = new ArrayList<Task>();
             for (Task element : dataAccessObject.getAll()) {
                 if (predicate.test(element)) {
-                    super.addElement(element);
+                    tasks.add(element);
                 }
             }
+            addAll(tasks);
         } catch (DataAccessException e) {
             e.printStackTrace();
             ErrorDialog.show("Error when querying all elements.", e);