preferences: setting invalid preference value results in unresponsive application

[vince-fugnitto]

Bug Description:

When a preference is set with an invalid value, the application will become unresponsive.
The value tested with is later used by monaco which may cause the actual issue.

The console is flooded with the following repeating error:

errors.ts:28 Uncaught Error: Invalid string length

RangeError: Invalid string length
    at IndentGuidesOverlay.prepareRender (indentGuides.ts:144)
    at ContentViewOverlays.ViewOverlays.prepareRender (viewOverlays.ts:125)
    at View._actualRender (viewImpl.ts:412)
    at viewImpl.ts:360
    at safeInvokeNoArg (viewImpl.ts:552)
    at View._renderNow (viewImpl.ts:360)
    at View._flushAccumulatedAndRenderNow (viewImpl.ts:230)
    at View._onRenderScheduled (viewImpl.ts:356)
    at AnimationFrameQueueItem.execute (dom.ts:210)
    at animationFrameRunner (dom.ts:250)
    at errors.ts:28

Steps to Reproduce:

1. start the application with theia as a workspace.
2. open the dev console to view logs.
3. open the settings.json file for the user or workspace scope.
4. add "editor.tabSize": 2 if it doesn't already exist - the application should work fine.
5. update the value to "editor.tabSize": "true" - the application becomes unresponsive, and the default value is not used.