struct StructWithArray {
    var arrayInStruct: [Int]
    
    init() {
        arrayInStruct = [Int]()
    }
}

var structs = [StructWithArray()]
// playground crashes when using this line
structs[0].arrayInStruct.append(3)
// this works just fine
let count = structs[0].arrayInStruct.count
structs[0].arrayInStruct[count] = 3