The three first tutorials on making a new property editor all linked to ...

..."Get the source", which was linked to the following location

https://github.com/umbraco/Umbraco-CMS/tree/7.0.0/src/Umbraco.Web.UI.Client/src/packages/MarkdownEditor

This location doesnt exist any more, and the package seems to have been moved from the project at some point in time.
"Fixed" by removing the part that links to the missing package. Another fix could be to restore the package