Issue with In-App Purchase - objective-c

Question

I'm testing my in-app purchase in sandbox mode but I dun understand why the in-app purchase alert box keeps popping up even though i've clicked Buy...
On the second time it pops up after i clicked 'Buy', i clicked 'Cancel' and there was an error:
Thread 1:EXC_BAD_ACCESS (code1, address=0x6000008)
My .m:
-(void)updateCredit{
//Adding to plist
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *path = [documentsDirectory stringByAppendingPathComponent:#"data.plist"];
NSMutableArray *currPlist = [[NSMutableArray alloc] initWithContentsOfFile: path];
NSString *lastEx = [currPlist objectAtIndex:0];
int lastScore = [[currPlist objectAtIndex:1] intValue];
int numberOfTries = [[currPlist objectAtIndex:2] intValue];
int totalScore = [[currPlist objectAtIndex:3] intValue];
int avg = [[currPlist objectAtIndex:4] intValue];
int credit = [[currPlist objectAtIndex:5] intValue];
credit += 10;
NSMutableArray *updatePlist = [[NSMutableArray alloc] init];
[updatePlist addObject:lastEx];
[updatePlist addObject:[NSNumber numberWithInt:lastScore]];
[updatePlist addObject:[NSNumber numberWithInt:numberOfTries]];
[updatePlist addObject:[NSNumber numberWithInt:totalScore]];
[updatePlist addObject:[NSNumber numberWithInt:avg]];
[updatePlist addObject:[NSNumber numberWithInt:credit]];
[updatePlist writeToFile: path atomically:YES];
}
-(void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions{
for (SKPaymentTransaction *transaction in transactions) {
switch (transaction.transactionState) {
case SKPaymentTransactionStatePurchasing:
break;
case SKPaymentTransactionStatePurchased:
[[SKPaymentQueue defaultQueue]finishTransaction:transaction];
[self updateCredit];
[self dismissModalViewControllerAnimated:YES];
break;
case SKPaymentTransactionStateRestored:
[[SKPaymentQueue defaultQueue]finishTransaction:transaction];
break;
case SKPaymentTransactionStateFailed:
if (transaction.error.code != SKErrorPaymentCancelled) {
UIAlertView *alert2 = [[UIAlertView alloc]initWithTitle:#"Error" message:#"Error updating credit" delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil];
[alert2 show];
}
[[SKPaymentQueue defaultQueue]finishTransaction:transaction];
break;
default:
break;
}
}
}