Trying Error Handling, getting "SIGABRT" error

I'm doing a little project to help myself understand error handling a bit better. In my app there are three input fields to collect the users name. First is for the first name, second field is for the middle name and so on,

When the user presses enter, I want it to show the concatenated fullname that was entered.

I am trying error handling to make sure that the first and last name fields are populated.

When the "Enter" button is pushed, I get a "SIGABRT" error even if there is nothing in the input fields. The top of the error says "unrecognized selector sent to instance"

Here is a chunk of my view controller...

@IBAction func EnterButton() throws {

    FeedbackImage.image = #imageLiteral(resourceName: "cross-out-mark")

    guard FirstName.text != nil else {
        FeedbackImage.image = #imageLiteral(resourceName: "cross-out-mark")
        throw Errors.InvalidData("There is no first name")
    }

    guard LastName.text != nil else {
        FeedbackImage.image = #imageLiteral(resourceName: "cross-out-mark")
        throw Errors.InvalidData("There is no last name")
    }

    let firstName = FirstName.text!
    let middleName = MiddleName.text!
    let lastName = LastName.text!

    let fullName = "\(firstName) \(middleName) \(lastName)"
    FullName.text = fullName

}

At the top of the view controller class I created an enum for the error...

enum Errors: Error {
    case InvalidData(String)
}

Can someone help me figure out what's going on??