Delete Tasks that do not belong to any series

backend/src/main/java/core/services/TaskService.java

@@ -90,7 +90,9 @@ public class TaskService {
     public void deleteTask(Task task) {
         task.getTaskgroup().getTasks().remove(task);
         taskgroupRepository.save(task.getTaskgroup());
-        taskSeriesService.deleteTaskSeriesItem(task);
+        if(task.getTaskSerieItem() != null) {
+            taskSeriesService.deleteTaskSeriesItem(task);
+        }
         taskRepository.delete(task);
 
     }