Reinstated the null check for TaskModel

core/src/main/java/com/netflix/conductor/core/dal/ExecutionDAOFacade.java

@@ -449,7 +449,9 @@ public List<Task> getTasksForWorkflow(String workflowId) {
 
     public TaskModel getTaskModel(String taskId) {
         TaskModel taskModel = getTaskFromDatastore(taskId);
-        populateTaskData(taskModel);
+        if (taskModel != null) {
+            populateTaskData(taskModel);
+        }
         return taskModel;
     }