You missed a step:
3a. On the Scripts tab, select one of the thumbnail images.
I see this with the Sept 02 5.1.0-prealpha appimage.
Terminal output:
kf5.ki18n: Trying to convert empty KLocalizedString to QString.
kf5.ki18n: Trying to convert empty KLocalizedString to QString.
Dialog “krita/generators/seexpr” has a widget without an object name: QScrollBar(0x30a1bb10)
Dialog “krita/generators/seexpr” has a widget without an object name: QScrollBar(0x309e33f0)
Segmentation fault
Looks like a bug report is needed
I don’t see a duplicate script (with a different name) created with the Sept 02 5.1.0-prealpha. I see it being renamed.
I do not think so. I think if I clear the thumbnail krita should replace it with white (or any other color) or predefined icon telling that this script is without a thumbnail.
If I follow your steps exactly, then on the Options tab the ‘Save New SeExpr Preset’ button is greyed out.
I need to first select one by clicking on it in the Scripts tab.