D24006: Remove the usage of "null backend"

[Alexander Semke]

asemke created this revision.
asemke added reviewers: sirgienko, filipesaraiva.
Herald added a project: KDE Edu.
Herald added a subscriber: kde-edu.
asemke requested review of this revision.

REVISION SUMMARY
  In b45af5e4de0cf283bb06544f367be9aa242e90a3 <https://phabricator.kde.org/R55:b45af5e4de0cf283bb06544f367be9aa242e90a3> the NullBackend class was removed. However, there is still some logic in the code making use of this "backend" which crashes Cantor when opening of a project file at the moment. This patch refactors the logic a bit and removes the remaining usage of the null backend completely.

REPOSITORY
  R55 Cantor

REVISION DETAIL
  https://phabricator.kde.org/D24006

AFFECTED FILES
  src/cantor.cpp
  src/cantor.h
  src/cantor_part.cpp
  src/main.cpp
  src/worksheet.cpp
  src/worksheet.h

To: asemke, sirgienko, filipesaraiva
Cc: kde-edu, narvaez, apol
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