SUMMARY
This crash does not happen with the 4.3.0 appimage
It happens with the Jun 29 5.0.0 prealpha (git 689fe92) and the Jun 29 4.3.1 alpha (git ccbadb2) appimages.

It happens when using the Move tool on a transform mask.

STEPS TO REPRODUCE
1. Paint a 'blob' on a paint layer.
2. Add a transform mask to the layer.
4. Select the transform mask then click the blob with the Move tool.
NOTE: A crash

OBSERVED RESULT
See Steps To Reproduce.
NOTE: With 4.3.0, using the Move tool on a transform mask does not give 'sensible' results but it doesn't crash.

EXPECTED RESULT
No crash.
Using the Move tool on a transform mask is probably something that should not be done. If so, the use of the Move tool should be blocked if you try to do that.

SOFTWARE/OS VERSIONS
Krita

 Version: 5.0.0-prealpha (git 689fe92)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-9-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10
  Desktop: MATE

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 750 Ti/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 440.82" 
  Shading language:  "4.60 NVIDIA" 
  Requested format:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
  Current format:    QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
     Version: 4.6
     Supports deprecated functions true 
     is OpenGL ES: false 

QPA OpenGL Detection Info 
  supportsDesktopGL: true 
  supportsOpenGLES: true 
  isQtPreferOpenGLES: false