What this error is invalid context 0x0?

I wrote the following code in ViewDidLoad

// Implement viewDidLoad to do additional setup after loading the view, typically     from      a nib.
     -(void)viewDidLoad {
    [super viewDidLoad];

NSString *str = [NSString stringWithFormat:@"Veer"];

CGColorSpaceRef rgbColorspace = CGColorSpaceCreateDeviceRGB();
CGFloat values[4] = {55.0, 0.0, 0.0, 1.0}; 
CGColorRef glowColor = CGColorCreate(rgbColorspace, values); 
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetShadowWithColor( context, CGSizeMake( 5.0, 0.0 ), 20.0f, glowColor);   
[str drawAtPoint:CGPointMake(10.5f, 0.5f) withFont:[UIFont      fontWithName:@"Helvetica" size:100.0f]];    
UILabel *label = [[UILabel alloc] initWithFrame:CGRe开发者_如何学编程ctMake(60, 200, 250, 100)];
[label setBackgroundColor:[UIColor clearColor]];
label.font=[UIFont fontWithName:@"DBLCDTempBlack" size:50.0];
label.textColor = [UIColor colorWithRed:0.0 green:1.0 blue:0.0 alpha:1.0];
[label setText:str];

[self.view addSubview:label];
  }

In Console I saw as

 Testing Clock[3286] <Error>: CGContextSetStyle: invalid context 0x0
 Testing Clock[3286] <Error>: CGContextSetFont: invalid context 0x0
 Testing Clock[3286] <Error>: CGContextSetTextMatrix: invalid context 0x0
 Testing Clock[3286] <Error>: CGContextSetFontSize: invalid context 0x0
 Testing Clock[3286] <Error>: CGContextSetTextPosition: invalid context 0x0
 Testing Clock[3286] <Error>: CGContextShowGlyphsWithAdvances: invalid context 0x0

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You should move that custom drawing code out of viewDidLoad and move it to drawRect.

You will also need to subclass the view if you want to implement your own drawRect method.

Checkout this example: http://ykyuen.wordpress.com/2010/08/27/iphone-uiview-with-border/

---

You just have to check if context exist before drawing :

CGContextRef context = UIGraphicsGetCurrentContext();
if (context) {
    // Do your stuff
}