Cantor is unable to initialize installed sage backend

[Norbert]

Steps to reproduce:
1. Install Cantor from official repository

    sudo apt-get install cantor cantor-backend-sage

2. Install SageMath 7.5.1~aimsppa5 from their PPA:

    sudo add-apt-repository ppa:aims/sagemath
    sudo apt-get update
    sudo apt-get install sagemath-upstream-binary-full

3. Make sure that SageMath is working from terminal
4. Launch Cantor from terminal with `cantor` and select Sage backend at start

Expected results:
Cantor started Sage backend, and user is able to work with it

Actual results:
Cantor started Sage backend, but user is unable to work with it. It shows window "Initializing Session" and messages in the terminal:

$ cantor
Object::disconnect: Unexpected null parameter
cantor(4104)/kdeui (kdelibs): Attempt to use QAction "" with KXMLGUIFactory!

Note:
this bug is inspired by AskUbuntu question (https://askubuntu.com/q/996535/66509).

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: cantor-backend-sage 4:4.13.3-0ubuntu0.1
ProcVersionSignature: Ubuntu 3.13.0-139.188-generic 3.13.11-ckt39
Uname: Linux 3.13.0-139-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.27
Architecture: amd64
CurrentDesktop: KDE
Date: Wed Jan 17 16:03:35 2018
SourcePackage: cantor
UpgradeStatus: No upgrade log present (probably fresh install)

[Sirgienko Nikita (mmmm98)]

This bug also mention on KDE bugs:
https://bugs.kde.org/show_bug.cgi?id=375467
You can subscribe on it and, if this bug will be fixed, you take notification (but also notification for new comments)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.