I've found a strange fault with Krita 4.0.0 appimage running on Debian 9, with an associated crash condition, just in time for Easter.
The crash and fault do not happen with the 3.3.3 appimage. I'd like some feedback in case I'm missing something obvious, before I raise a bug report. I'll describe the crash first because it's fairly simple to do..

Crash:
Start Krita, then New image Create.
Then Add a Vector Layer then Add another Vector layer.
Choose the Freehand Path Tool and draw a curve in the lower vector layer. It will be drawn in the selected colour with the brush preset that is selected (or a vector approximation to it).
Select the upper vector layer and try to draw a curve in it. It can't be done. (You can draw a vector shape or a Bezier curve if you want to, but don't bother for now.)
Right click the upper vector layer and click Merge With Layer Below.
Do File -> Close and say 'No' to saving the file.
Do File -> New
Krita will crash when Create is pressed.


Fault:
The fault involves being unable to draw a Freehand Path in a vector layer but it can vary depending on the previous actions.
As an example, I found that drawing a freehand path in a paint layer enabled drawing of freehand paths in vector layers. In another test, I found that drawing other vector shapes first allowed me draw a vector freehand path.
It's complicated and not always repeatable.