a strange swift bug. “thread return -x” How to deal with it

I executed the code as below. it was occurred a error with the title as you can see! I don't how to handle it

import UIKit

var str = "Hello, playground"

print("hello world!")

class Solution {
    func findMedianSortedArrays(_ nums1: [Int], _ nums2: [Int]) -> Double {



        let a = nums1.first!
        let b = nums1.last!
        let c = nums2.first!
        let d = nums2.last!

        if c >= a && c >= b {
            let conbineArray = nums1 + nums2
            let arrayCount = nums1.count + nums2.count
            if arrayCount % 2 == 0  {
                return Double((conbineArray[(arrayCount / 2) - 1] + conbineArray[arrayCount / 2]) / 2)
            } else {
                return Double(conbineArray[ ((arrayCount + 1) / 2) - 1])
            }
        }

        if c <= a && d <= a {
            let conbineArray = nums2 + nums1
            let arrayCount = nums2.count + nums1.count
            if arrayCount % 2 == 0 {
                return Double((conbineArray[(arrayCount / 2) - 1] + conbineArray[arrayCount / 2]) / 2)
            } else {
                return Double(conbineArray[ ((arrayCount + 1) / 2) - 1])
            }
        }

        let totalArray = nums1 + nums2
        let sortedArray = totalArray.sorted(by: < )
        if sortedArray.count % 2 == 0 {
            return Double((sortedArray[(sortedArray.count / 2) - 1] + sortedArray[sortedArray.count / 2]) / 2)
        } else {
            return Double(sortedArray[ ((sortedArray.count + 1) / 2) - 1])
        }

    }
}

let b:[Int] = []
let a = [1]

let s = Solution()
s.findMedianSortedArrays(a, b)

The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation.