Error after installing theia-yang-extension from npm

[JanKoehnlein]

• Run theia from the sources (electron or browser)
• install theia-yang-extension using the extension manager
• reload

The screen stays black. On the console I see

index.js:50 Error: Cannot apply @injectable decorator multiple times.
    at injectable.js:8
    at DecorateConstructor (Reflect.ts:1090)
    at Object.decorate (Reflect.ts:284)
    at _decorate (decorator_utils.js:42)
    at Object.decorate (decorator_utils.js:55)
    at Object.defineProperty.value (widget.ts:13)
    at __webpack_require__ (bootstrap 0c054091550f5e565229:688)
    at fn (bootstrap 0c054091550f5e565229:109)
    at Object.m (index.ts:8)
    at __webpack_require__ (bootstrap 0c054091550f5e565229:688)

The target is the class Widget.

[kittaakos]

See: eclipse-theia/theia#752