I used to be able to open *.sql files in my Eclipse text editor. For some reason that stopped working this morning:

    Invalid menu handle.

    Problems opening an editor... unable to open external editor.

To fix this behavior for ALL files of a certain extension:

    Window > Preferences -

... and on the Preferences Tree:

    General > Editors > File Associations -

... add your File Type extension, for example *.sql

... and finally add your Associated Editor. I chose "internal editors" - "text editor"

You can also change the behavior "temporarily" for a single file. Right click the file, click "Open With..." and select the editor...