Exception thrown when bpmndi:BPMNPlane elements does not reference a process id

See method BpmnParse#parseBPMNPlane:
String processId = bpmnPlaneElement.attribute("bpmnElement");

--> the bpmnElement attribute does not necessarily have to reference a process id. It can also reference a collaboration id. See example below, created with the modeler (full XML file attached):

<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" ...>
<process id="make-test-process" isExecutable="false" name="order process">
...
</process>

<collaboration id="sid-c3b705f9-79f9-4bb2-9f27-f9768c12ea66">
<participant id="pool3" name="order process" processRef="make-test-process"/>
</collaboration>

<bpmndi:BPMNDiagram id="sid-361594e9-cc3a-4726-b5ba-f6b232b74d1b">
<bpmndi:BPMNPlane bpmnElement="sid-c3b705f9-79f9-4bb2-9f27-f9768c12ea66" id="sid-f9262834-7a61-4df1-9ea0-dcf3b8824f40">
...
</bpmndi:BPMNPlane>
</bpmndi:BPMNDiagram>