SUMMARY
The image split option is throwing error, when trying to do so.

STEPS TO REPRODUCE
1. Open a project with a long image, (mine is 800 x 10000)
2. go to Image -> Image Split -> Choose any type to save (I tried with jpeg and kra types) -> Choose a folder to save the split images in
3. Horizontal lines 10, Vertical lines 0, give prefered prefix and click apply.

OBSERVED RESULT

An error box opens up with the following message :

SAFE ASSERT (krita): "!sanityCheckPointer.isValid()" in file /home/appimage/workspace/Krita_Release_Appimage_Build/krita/libs/ui/KisDocument.cpp, line 500

Then it asks to ignore or abort. Clicking on ignore just resulted in the same popup again and again. Clicking on abort closed Krita.

EXPECTED RESULT

The single long image, will be split and saved into 10 images in the selected folder.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION

Krita
  Version: 4.1.7

Qt
  Version (compiled): 5.10.0
  Version (loaded): 5.10.0

OS Information
  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.4.0-145-generic
  Pretty Productname: Ubuntu 16.04.6 LTS
  Product Type: ubuntu
  Product Version: 16.04

OpenGL Info
  **OpenGL not initialized**