Could You Explain Why You Made This?
Being able to link to a document simplifies answering questions about how to do something in the editor. A text-based tutorial has the advantage that it can be updated more easily than a video. Besides, I would have benefited a lot from a guide like this if it had existed when I made my first level. When I wanted to achieve something, I often looked at other levels and copied from there. Having everything in one place simplifies finding the information you’re looking for.
I found that if i must have selected sth from the object category.
Seems like a bug.
See
https://github.com/SuperTux/supertux/issues/1169