WebException.Response can be null  ( this is the case, when 'WebException.Status != WebExceptionStatus.ProtocolError' ). Add a null check.
WebException.Response can be null  ( this is the case, when 'WebException.Status != WebExceptionStatus.ProtocolError' ). Add a null check.

aluxnimm/outlookcaldavsynchronizer,DoCode/Outlook-CalDav-Synchronizer