fix 'Access to undefined property' errors, fixes #1295

lib/activities/CoursewareContext.php

...trictedadmission/ParticipantRestrictedAdmission.class.php

lib/calendar/CalendarColumn.class.php

lib/calendar/CalendarImport.class.php

lib/calendar/CalendarParser.class.php

lib/calendar/CalendarView.class.php

lib/calendar/CalendarWeekView.class.php

lib/calendar/lib/ErrorHandler.class.php

lib/classes/ContentBar.php

lib/classes/ContentGroupMenu.php

lib/classes/DataFieldEntry.class.php

lib/classes/DataFieldSelectboxEntry.class.php

lib/classes/DatabaseObject.class.php

lib/classes/MultiPersonSearch.class.php

lib/classes/MvvQuickSearch.php

lib/classes/QuickSearch.class.php

lib/classes/SemBrowse.class.php

lib/classes/StudipRangeTree.class.php

lib/classes/StudipSemRangeTreeViewSimple.class.php

lib/classes/StudipSemSearch.class.php