Can you please answer some more questions to help us investigate further:
1) Can you please share your OS details and version ?
2) Are you trying to open a existing struts.xml file (or) a struts.xml file which is newly created ? If you are opening a existing file then try opening struts.xml file which is newly created and check whether you see the same issue ?
3) Can you please switch to a new workspace and check whether you can replicate the issue there ?
4) Clear the contents of the .log file located at <Workspace dir>/.metadata/.log, try opening the struts.xml file and attach the .log file here.