BUG: Reference layout manager only if it is available

This will the following error happening when running py generic test:

test_widgetRepresentation (qSlicerCornerAnnotationModuleGenericTest.qSlicerCornerAnnotationModuleGenericTest) ... Traceback (most recent call last):
  File "/Users/ayamada/Documents/Github_CornerAnnotationPy-build/lib/Slicer-4.3/qt-scripted-modules/CornerAnnotation.py", line 425, in setup
    self.redRenderer = slicer.app.layoutManager().sliceWidget('Red').sliceView().renderWindow().GetRenderers().GetFirstRenderer()
AttributeError: 'NoneType' object has no attribute 'sliceWidget'

See http://slicer.cdash.org/testDetails.php?test=5712110&build=268460