Created attachment 76411 [details]
stacktrace

Probable regression from recent changes to properties module. Prevented me from opening a project.properties file at
all. Maybe it is getting confused by project.xml?

Easily reproducible. Just create a project, Go to File, "project.properties", Enter.

BTW

            files = new FileObject[listFileObjects.size()];
            int index = 0;
            for (FileObject file : listFileObjects) {
                files[index++] = file;
            }

can be simplified to

            files = listFileObjects.toArray(new FileObject[listFileObjects.size()]);

Clearly the following code is wrong:

            for (FileObject file : parent.getChildren()) {
                fName = file.getName();
                if (fName.equals(baseName) && file.isValid()) {
                    listFileObjects.add(0,file);
                }

Should have:

if (!file.hasExt("properties")) {
  continue;

Even given that, getNthEntry should be written more conservatively and just quietly return null in case DataObject.find
returns some other kind of DataObject; I have in fact heard on the mailing lists of people writing specialized data
subtypes that match certain kinds of *.properties files.
}

Thanks for comments.

changeset 8f8221d23063

Commenting out the DataObjectNotFoundException.printStackTrace() where you did would not prevent the ClassCastException
from being reported as an uncaught exception.

Yes, it is not prevent. But as you wrote the problem is that project.xml was taken into account.
And it is fixed. So only PropertiesDataObject instances can get there now.

Added DataObject instanceof PropertiesDataObject check 
changeset 9f4151676c90

As I wrote earlier, it is possible (when using 3rd-party modules) for a *.properties file to not be a
PropertiesDataObject. 9f4151676c90 looks right.