iOS error that I can't replicate - ios

Question

I have a revision quiz app in the App store that has been receiving around 10 downloads per day for the last 3 months. It's quite well used and for the most part has good reviews. However, recently I've had two 1 star reviews because people are complaining that the "Review" screen is blank after the quiz ends.
I've tried this on iPhone 5/5c/5s and it works perfectly well. These are the only devices I have access to, but I've also run the simulator for iPad and both iPhone 6 versions and I can't replicate the problem.
The flow is RootController->QuizController->ResultsController->ReviewController. The quiz controller has an object called GameManager as an instance variable. This tracks the user answers and questions as the quiz progresses, and is passed from the Quiz to the Results and then to the Review section during segues.
The functions that pass the object are:
//From Quiz to Results
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
// Make sure your segue name in storyboard is the same as this line
if ([[segue identifier] isEqualToString:#"quiz2results"])
{
// Get reference to the destination view controller
ResultsController *vc = [segue destinationViewController];
// Pass any objects to the view controller here, like...
[vc setManager:_manager];
}
}
//From Results to Review
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
// Make sure your segue name in storyboard is the same as this line
if ([[segue identifier] isEqualToString:#"results2review"])
{
// Get reference to the destination view controller
ReviewController *vc = [segue destinationViewController];
// Pass any objects to the view controller here, like...
[vc setManager:_manager];
}
}
Then in the Review controller the following code is executed:
-(void)setReviewText{
NSArray* questions = [[NSArray alloc] init];
questions = [_manager getAllQuestions];
NSMutableArray* userAnswer = [[NSMutableArray alloc] init];
userAnswer = [self answersAttributed];
if (userAnswer == nil)
{
_output.text = #"Error";
}
NSMutableAttributedString *mutableAttString = [[NSMutableAttributedString alloc] init];
int i = 0;
NSAttributedString *linebreak =[[NSAttributedString alloc] initWithString:#"\n"];
for (Question* q in questions)
{
if (i == [userAnswer count])
break;
NSAttributedString *qtext =[[NSAttributedString alloc] initWithString:q.questionText];
NSAttributedString *ctext =[[NSAttributedString alloc] initWithString:#"Correct Answer:\n"];
NSAttributedString *atext =[[NSAttributedString alloc] initWithString:q.answerText];
NSAttributedString *ytext =[[NSAttributedString alloc] initWithString:#"Your Answer:\n"];
NSAttributedString *utext =[[NSAttributedString alloc] initWithAttributedString:userAnswer[i]];
[mutableAttString appendAttributedString:qtext];
[mutableAttString appendAttributedString:linebreak];
[mutableAttString appendAttributedString:ctext];
[mutableAttString appendAttributedString:atext];
[mutableAttString appendAttributedString:linebreak];
[mutableAttString appendAttributedString:ytext];
[mutableAttString appendAttributedString:utext];
[mutableAttString appendAttributedString:linebreak];
[mutableAttString appendAttributedString:linebreak];
i++;
}
NSAttributedString* outText = [[NSAttributedString alloc] init];
outText = [mutableAttString mutableCopy];
_output.attributedText = outText;
[_output setFont:[UIFont preferredFontForTextStyle:UIFontTextStyleBody]];
}
-(NSMutableArray*)answersAttributed
{
NSMutableArray* ansAtt = [[NSMutableArray alloc] init];
NSArray* questions = [[NSArray alloc] init];
questions = [_manager getAllQuestions];
NSArray* userAnswers = [[NSArray alloc] init];
userAnswers = [_manager getAllUserAnswers];
if ([questions count] != [userAnswers count])
{
return ansAtt;
}
int i = 0;
for (NSString* userAnswer in userAnswers )
{
if([[questions[i] answerText] isEqualToString:userAnswer] )
{
NSDictionary *attributes = #{NSBackgroundColorAttributeName:[UIColor greenColor]};
NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:userAnswer attributes:attributes];
[ansAtt addObject:attrString];
}
else
{
NSDictionary *attributes = #{NSBackgroundColorAttributeName:[UIColor redColor]};
NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:userAnswer attributes:attributes];
[ansAtt addObject:attrString];
}
i++;
}
return ansAtt;
}
If anybody could shed some light on this I'd be delighted. The cause of this problem has alluded me so far.