Fix miscellaneous issues with QuadView.

+ Fixed segfault when a source shown in the view was deleted and if that
source had a scalar bar visible in the view. The logic was ignoring
non-data representations entirely. Fixed that.

+ Fixed errors during startup due to missing text for annotation. This
was causing OpenGL errors are VTK updates.

Change-Id: I8dd2d5d2fec4a09155c9c1ed86fbaeeca1f7941e
Merge topic 'misc_quad_view_fixes' into next

ed10ca5d Fix miscellaneous issues with QuadView.