fix: RAW_BPMN_RESOURCE is not defined

[nitram509]

Just a minor thing, but since the app.js loads in every view,
there was a JS error, when the screen model does not contain the required field, like in 'errors' for example.

Error was:

app.js:714 Uncaught ReferenceError: RAW_BPMN_RESOURCE is not defined
          at checkIfBpmnExistsOrDisableDownloadButton (app.js:714:5)

[saig0]

@nitram509 thank you for your contribution. 🎉 I'll have a look at your PR in the next few days. 👀

[saig0]

@nitram509 👍