Prevent PHP-Warnings, closes #1764

app/controllers/admin/datafields.php

app/controllers/admin/sem_classes.php

app/controllers/calendar/schedule.php

app/controllers/calendar/single.php

app/controllers/contents/courseware.php

app/controllers/oer/mymaterial.php

app/controllers/terms.php

app/views/calendar/contentbox/_termin.php

app/views/calendar/schedule/index.php

app/views/calendar/single/year.php

app/views/course/wizard/steps/basicdata/index_studygroup.php

app/views/course/wizard/steps/studyareas/index.php

app/views/my_studygroups/_course.php

app/views/oer/market/index.php

app/views/oer/mymaterial/edit.php

lib/classes/StudipRangeTree.class.php

lib/classes/StudipRangeTreeViewAdmin.class.php

lib/classes/calendar/SingleCalendar.php

lib/classes/coursewizardsteps/StudyAreasWizardStep.php

lib/classes/sidebar/RoomSearchWidget.class.php