Error:
Could not open the editor: The editor class could not be instantiated. This
When I faced this issue. I simple updated the eclipse and it resolve the problem.
I've got the same problem though my eclipse is much newer - 4.2 (Juno). For me the solution was to remove <workspace>/.metadata/.plugins/org.eclipse.core.resources/.snap
. Afterwards I found this bug in eclipse.org: Bug 178988 - Crash at startup due to ".snap" files.
Hope this will help somebody.