To fix the NullSourceAcess issue add this to the /Config/DefaultEditorSettings.ini
[/Script/SourceCodeAccess.SourceCodeAccessSettings]
PreferredAccessor=KDevelop
If the ini does not exist create it. The default is the Null one because you never know what people have installed on their machines.
As for the conversion. If you are sure that you are using the same engine version you can skip the conversion(it is under more options in that window)
The crash is curious, and would need to see the crash log to be able to help out with that, but on 4.26 there is currently an issue with vulkan defragmenter, so I tend to recommend to disable that in Config/DefaultEngine.ini
[SystemSettings]
r.Vulkan.EnableDefrag=0
One last bit… since you have a c++ project you might want to run the compilation from the CLI and see where it fails something like this will work
<PATH TO ENGINE>/Engine/Build/BatchFiles/Linux/Build.sh <PROJECT NAME>Editor Linux Development -Project=<PATH TO PROJECT>/<PROJET NAME>.uproject
Obviously fill in the blanks