Charts: Can't set hole color to nil for Pie Chart

For a transparent hole, it is recommended to:

*Note: Use holeTransparent with holeColor = nil to make the hole transparent.*```

However, the following set up causes a crash:

pieChart.holeTransparent = true
pieChart.holeColor = nil

Looking at the implementation, the holeColor is explicitly unwrapped, presumably because of the check for nil that precedes it. However, the explicit unwrapping is not in scope of the check:

if (holeColor !== nil && holeColor != UIColor.clearColor())
            {
                // draw the hole-circle
                CGContextSetFillColorWithColor(context, holeColor!.CGColor)
                CGContextFillEllipseInRect(context, CGRect(x: center.x - holeRadius, y: center.y - holeRadius, width: holeRadius * 2.0, height: holeRadius * 2.0))
            }

            // only draw the circle if it can be seen (not covered by the hole)
            if (transparentCircleRadiusPercent > holeRadiusPercent)
            {
                let alpha = holeAlpha * _animator.phaseX * _animator.phaseY
                let secondHoleRadius = radius * transparentCircleRadiusPercent

                // make transparent
                CGContextSetFillColorWithColor(context, holeColor!.colorWithAlphaComponent(alpha).CGColor)

                // draw the transparent-circle
                CGContextFillEllipseInRect(context, CGRect(x: center.x - secondHoleRadius, y: center.y - secondHoleRadius, width: secondHoleRadius * 2.0, height: secondHoleRadius * 2.0))
            }

Temporary workaround:

pieChart.holeColor = UIColor.clearColor()