Page jsff not a source file warning

Warning: java.lang.IllegalArgumentException: The file file:<directory listed here>\DepartmentsView.jsff is not a source file

https://blogs.oracle.com/jdevotnharvest/entry/warning_java_lang_illegalargumentexception_the

<!-- 
  * WORK AROUND
  * randomly, when editing files, compilation fails with an illegal argument exception
  * java.lang.IllegalArgumentException: The file <file name> .jsff is not a source file
  * To avoid this error, which initially has been reported as bug 12732652 for Facelets
  * I added the jsp-property-group entry below
 -->
 <jsp-config>
  <jsp-property-group> 
    <url-pattern>*.jsff</url-pattern>
    <is-xml>true</is-xml>
   </jsp-property-group>
  </jsp-config>
 <!-- 
  * END WORK AROUND
  -->