Process invalid

[paolasanmartinmurillo]

I've created a simple BPMN model and i recieve a warning. It seems that come from the exclusive gateway. It can't get the source or can't set the source but I don't know how to fix it.
This is the bug I recieve:
Invalid Process
Argument- 1 passed to
ProcessMaker\Nayra\Bpmn\Models\Flow::setSource() must implement interface ProcessMaker\Nayra\Contracts\Bpmn\FlowNodeInterface, instance of ProcessMaker\Nayra\Bpmn\Models\Flow given, called in /code/pm4/vendor/processmaker/nayra/src/ProcessMaker/Nayra/Storage/BpmnElement.php on line 146

And this is the BPMN model:

All the tasks have their own screen and this is how I have defined the gateway:

I haven't put anything in the flow variable because i don't want any variable to be changed.

Thanks.