Correctly set initial value for extent path in legacy UI

I'm attaching a picture to this ticket to hopefully show the problem that the customer pointed out to us.

In this picture, you can see the background of the image and notice that the "LUN-Exchange-mdb3" row is selected.

However, when the customer double-clicked that row the "Edit" dialog box pops up and if you look closely the "Device:" drop-down option has a different extent that is being auto-populated.

If you click "OK" in the Edit dialog box, it accepts the input but doesn't actually change the extent mapping (even though the "Edit" dialog box show it's changed). This causes confusion and actually makes it seem like your easily able to cause a production impacting change.