CGBitmapContextCreate return NULL

[wanghongli123]

CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef) & kCGBitmapAlphaInfoMask;

//Since iOS8 it's not allowed anymore to create contexts with unmultiplied Alpha info
if (bitmapInfo == kCGImageAlphaLast) {
    bitmapInfo = kCGImageAlphaPremultipliedLast;
}
if (bitmapInfo == kCGImageAlphaFirst) {
    bitmapInfo = kCGImageAlphaPremultipliedFirst;
}