switching to subprocess diagrams not working if BPMNDiagram element or id attribute is missing

[schattenk]

Tested on Windows using Camunda Modeler 5.4.1 and Multi-Diagram plug-in 2.0.2

When the BPMNDiagram element that corresponds to a sub processes is missing, or when the BPMNDiagram is present but its id attribute is missing, then switching to that diagram does not work using the plugin (but does when using Modeler itself).

I found this issue because I'm getting models from another modeling tool into Camunda. The import works without errors or warnings.

See attached examples

• no-diagram-element.txt: This is what I get from my colleagues. The BPMNDiagram element for the sub process is missing.
• no-id.txt: This is what I get if I edit the previous model and save it using Camunda Modeler. A BPMNDiagram element is added, but it has no id attribute.
  (Sorry for using txt extension, GitHub didn't allow uploading bpmn files).

In both cases, an empty button is shown in the "Global subprocesses configuration", but clicking it makes nothing happen. This seems to be a bug, because in Camunda Modeler, both models link to the diagram (click on the collapsed subprocess, a blue/white arrow appears, clicking it opens an empty diagram).