Only dereferencing context when valid in Cmiss_context_get_default_region and Cmiss_context_get_default_graphics_module.  https://tracker.physiomeproject.org/show_bug.cgi?id=3181
Only dereferencing context when valid in Cmiss_context_get_default_region and Cmiss_context_get_default_graphics_module.  https://tracker.physiomeproject.org/show_bug.cgi?id=3181

cmiss/cmgui,cmiss/cmgui