Select marker returns null if the marker is invalid and having a null range in the selections array causes issues in the editor API, which is why that spec is failing.

Something like this is needed:

```coffee
setTabStopIndex: (@tabStopIndex) ->
    @settingTabStop = true
    markerSelected = false

    selections = []
    for marker in @tabStopMarkers[@tabStopIndex]
      if selection = @editor.selectMarker(marker)
        selections.push(selection)

    if selections.length > 0
      @editor.setSelectedBufferRanges(selections)
      markerSelected = true

    @settingTabStop = false
    markerSelected
```