Save as button should be disabled for EmptyTab

[barmac]

Describe the Bug

Save as button is enabled for empty tab and it causes an unhandled Promise rejection on backend. The issue did not occur in Camunda Modeler 2.2.4.

Steps to Reproduce

1. Open Camunda Modeler and close all tabs.
2. Click on save as button.

There is an unhandled Promise rejection on backend, see log below:

(node:76526) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'path' of undefined
    at /Users/maciej.barelkowski/workspace/bpmn-io/camunda-modeler/app/lib/index.js:169:12
    at EventEmitter.<anonymous> (/Users/maciej.barelkowski/workspace/bpmn-io/camunda-modeler/app/lib/util/renderer.js:39:14)
    at EventEmitter.emit (events.js:182:13)
    at WebContents.<anonymous> (/Users/maciej.barelkowski/workspace/bpmn-io/camunda-modeler/node_modules/electron/dist/Electron.app/Contents/Resources/electron.asar/browser/api/web-contents.js:285:13)
    at WebContents.emit (events.js:182:13)
(node:76526) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:76526) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
app:main window focused

Expected Behavior

Button should be disabled and no message to backend should be sent.

Environment

• OS: MacOS
• Camunda Modeler Version: master

[pinussilvestrus]

Closed via #1285