[plugin] Failed to show text document for already opened document

[akosyakov]

Reproducible with #6044 (comment)

winjs.base.js:1230 Uncaught (in promise) Error: Failed to show text document [object Object]
    at Object.<anonymous> (/workspace/theia/packages/plugin-ext/lib/plugin/plugin-context.js:246)
    at step (/workspace/theia/packages/plugin-ext/lib/plugin/plugin-context.js:47)
    at Object.next (/workspace/theia/packages/plugin-ext/lib/plugin/plugin-context.js:28)
    at fulfilled (/workspace/theia/packages/plugin-ext/lib/plugin/plugin-context.js:19)
    at process._tickCallback (/internal/process/next_tick.js:68)

Also [object Object] is not very informative.