title: "<Error>: CGContextDrawImage: invalid context 0x0"
body: <p>I have this code, which I found on here, which finds the color of a pixel in an image:</p>
  
  <pre><code>+ (NSArray*)getRGBAsFromImage:(UIImage*)image atX:(int)xx andY:(int)yy count:(int)count
  {
      NSMutableArray *result = [NSMutableArray arrayWithCapacity:count];
  
      // First get the image into your data buffer
      CGImageRef imageRef = [image CGImage];
      NSUInteger width = CGImageGetWidth(imageRef);
      NSUInteger height = CGImageGetHeight(imageRef);
      CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
      unsigned char *rawData = malloc(height * width * 4);
      NSUInteger bytesPerPixel = 4;
      NSUInteger bytesPerRow = bytesPerPixel * width;
      NSUInteger bitsPerComponent = 8;
      CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                                   bitsPerComponent, bytesPerRow, colorSpace,
                                                   kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
      CGColorSpaceRelease(colorSpace);
  
      CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
      CGContextRelease(context);
  
      // Now your rawData contains the image data in the RGBA8888 pixel format.
      int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;
      for (int ii = 0 ; ii &lt; count ; ++ii)
      {
          CGFloat red   = (rawData[byteIndex]     * 1.0) / 255.0;
          CGFloat green = (rawData[byteIndex + 1] * 1.0) / 255.0;
          CGFloat blue  = (rawData[byteIndex + 2] * 1.0) / 255.0;
          CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
          byteIndex += 4;
  
          UIColor *acolor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
          [result addObject:acolor];
      }
  
      free(rawData);
  
      return result;
  }
  </code></pre>
  
  <p>But at the line <code>CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);</code> it gives an error in NSLog of <code>&lt;Error&gt;: CGContextDrawImage: invalid context 0x0</code>. It doesn't crash the app, but obviously I'd prefer not to have an error there.</p>
  
  <p>Any suggests for this?</p>