Terminating app due to uncaught exception 'NSInvalidArgumentException' 5 - objective-c

Question

i am getting this error while running my app:
my code is:
- (IBAction)yes:(id)sender {
UIAlertView *msgbox=[[UIAlertView alloc]initWithTitle:#"Warrning" message:nil delegate:self cancelButtonTitle:#"Ok" otherButtonTitles:nil, nil];
if ([_usernametextbox.text length]>0)
{
if ([_passwordtextfield.text length]>0)
{
NSMutableString *ms=[[NSMutableString alloc]initWithFormat:url,_usernametextbox.text,_passwordtextfield];
NSURL *serviceurl=[[NSURL alloc] initWithString:ms];
NSError *error=nil;
NSData *data=[NSData dataWithContentsOfURL:serviceurl options:NSDataReadingUncached error:&error];
NSMutableDictionary *dic=[[NSMutableDictionary alloc]init];
NSString *jsondata=[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
SBJSON *parsedata=[[SBJSON alloc] init];
dic=[parsedata objectWithString:jsondata error:nil];
if([dic count]>0)
{
NSMutableString *st=[[NSMutableString alloc]initWithString:[dic objectForKey:#"loginResult:#""+"]]; //#" " withString:#"+"
if ([st isEqualToString:#"YES"])
{
UIAlertView * alert=[[UIAlertView alloc] initWithTitle:#"accept" message:#"correct username" delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil, nil];
[alert show];
}
else
{
UIAlertView * alert=[[UIAlertView alloc] initWithTitle:#"Error" message:#"Invalid UserName and Password" delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil, nil];
[alert show];
}
}
}
else
{
msgbox.message=#"Please Enter Password...";
[msgbox show];
}
}
else
{
msgbox.message=#"Please Enter Username...";
[msgbox show];
}
I get the error:
Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: '* -[NSConcreteData
initWithContentsOfURL:options:error:]: nil URL argument'
on the line:
NSMutableString *st=[[NSMutableString alloc]initWithString:[dic objectForKey:#"loginResult:#""+"]]; //#" " withString:#"+" if ([st isEqualToString:#"YES"])