setValue:forUndefinedKey:]:
You are getting the error because there is no property called loginButton. Check your .xib file. There is almost certainly a bad link to a FinalViewWithSending object (likely File's Owner) that specifies loginButton even though it doesn't exist in the class code.
Phillip Mills is correct.
The answer here, for anyone searching, is to edit the storyboard and remove any link to the undefined key specified. You have to edit the storyboard file in an external editor: Right click on the storyboard listing in the hierarchy and then click on "show in finder" or what have you. Open in a text editor, remove said links by searching, save and return to Xcode. No more issue.
This happens when you remove something from the view controller improperly.
Agreeing here - Phillip Mills is correct.
Someone mentioned opening the storyboard up in a text editor - this isn't necessary.
Just click on the storyboard, go to the Connections Inspector. If you've got a problematic outlet, you'll see an exclamation marker next to the outlet. Delete it.