Property cannot be marked @IBInspectable because its type cannot be represented in Objective-C

[FaizUlHassan123]

    @IBInspectable open var stepValue: Double? = nil {
    willSet(newValue) {
        if newValue != nil {
            assert(newValue! > 0, "NHRangeSlider: stepValue must be positive")
        }
    }
    didSet {
        if let val = stepValue {
            if val <= 0 {
                stepValue = nil
            }
        }

        updateLayerFrames()
    }
}

 @IBInspectable open var stepValue: Double? = nil {
            didSet {
        self.rangeSlider?.stepValue = stepValue
    }
}