1. You need to store the last selection in a file. Upon start of your program you load the file and use the stored content. (Usually that's what config files, support by those config file functions, are used for!)
2. Which warning? Where do you get that warning? After all it's just a warning and no error… (I would be more worried about the missing label of that control!)
The very large majority of LabVIEW developers don't have the Show Warnings checked on the Error List dialog. This is because it will nag sometimes about things that aren't important, like having a tab control on the UI that doesn't need to be wired to anything, but is still a warning. I'd highly suggest unchecking the Show Warnings checkbox in the View >> Error List dialog.
For other style like checking (like having labels on all controls) I'd suggest the VI analyzer which is now included in most versions of LabVIEW, and can scan for things like backwards wires, unlabeled or duplicate labeled controls, spelling errors, etc.