Working on my GSoC project, I noted that the generated Xtext Editor simply stopped to work. Looking at the logs I found the message:

!MESSAGE Unable to create editor ID org.eclipse.amalgam.tutorials.xtext.Droid: No editor descriptor for id org.eclipse.amalgam.tutorials.xtext.Droid

And when I launched the “Eclipse Application” run configuration, the resulting Eclipse warned:

Could not open the editor: No editor descriptor for id org.eclipse.amalgam.tutorials.xtext.Droid

After some time cursing Xtext, I discovered that in order to launch the “Eclipse Application” run configuration, you must to have all the generated projects opened in the package explorer (that means both .generator and .ui projects must be open).