fix: segfault

fix: segfault
 - fixes segfault when switching days in "Day View"
 - TODO: needs to be verified, 'cause it looks like
         it shouldn't work, but it does
   - when calling 'QAbstractItemModel::removeRows()' it returns false,
     but it prevents application from crash(segfault)
     - possible explanation is that the timing has changed and so the
       conditions for the segfault