Following scabanovich's comment in JBIDE-20761:

My concern is that the default editor is called if it is set for .json without a fallback for a case of failure of that unknown editor. We cannot be sure what editor a user may have set to be default for .json. That editor may be expecting some specific instance of editor input object and fail on what is provided. As it is, then json will not be opened at all, neither fancy nor plain text. However, if it is just temporal, till the expected contribution to jsdt, and that editor is going to be hardcoded to be opened, then no problem.

Until we can rely on Eclipse's upcoming JSON editor (in Neon), we should try to harden editor detection/opening