UITableViewCell is nil when not visible in a large UITableView (Xcode, iOS)

I've made a large UITableView with 27 static UITableViewCells. When selecting a UITableViewCell it should set its accessoryType to UITableViewAccessoryCheckmark and the last selected UITableViewCell's accessoryType to UITableViewAccessoryNone:

- (void)viewWillAppear:(BOOL)animated {
  [super viewWillAppear:animated];

  // get an NSInteger from NSUserDefaults

  [self.tableView selectRowAtIndexPath:[NSIndexPath indexPathForRow:savedInteger inSection:0] animated:NO scrollPosition:UITableViewScrollPositionTop];
  [self tableView:self.tableView didSelectRowAtIndexPath:[NSIndexPath indexPathForRow:savedInteger inSection:0]];
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
  // get the NSInteger again

  UITableViewCell *lastCell = [tableView cellForRowAtIndexPath:[NSIndexPath indexPathForRow:savedInteger inSection:0]];
  UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

  lastCell.accessoryType = UITableViewAccessoryNone;
  cell.accessoryType = UITableViewAccessoryCheckmark;

  // save indexPath.row in NSUserDefaults and do some other stuff

This works fine when you select a UITableViewCell that is near to the last selected UITableViewCell but when the last selected UITableViewCell is about 11 rows or so away, the UITableViewCell *lastCell is nil and the checkmark doesn't disappear.

Why does it behave like that and how can I fix it? Thanks in advance.

cellForRowAtIndexPath returns nil if cell is not visible or index path is out of range.
Therefore, check whether the currently selected cell is visible.

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {

    // get the NSInteger from NSUserDefaults
    NSIndexPath *previousSelection = [NSIndexPath indexPathForRow:savedInteger inSection:0];
    if ([tableView.indexPathsForVisibleRows containsObject:previousSelection]) {
        UITableViewCell *lastCell = [tableView cellForRowAtIndexPath:previousSelection];
        lastCell.accessoryType = UITableViewAccessoryNone;
    }

    // save indexPath.row in NSUserDefaults

    UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
    cell.accessoryType = UITableViewAccessoryCheckmark;

    // do other stuff
}

Also, in order to ensure that the table view cells are created with the desired accessoryType in the first place, set the accessoryType of the table view cell in cellForRowAtIndexPath.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    // get the table view cell and set the details

    cell.accessoryType = UITableViewAccessoryNone;

    // get the NSInteger from NSUserDefaults
    if (indexPath.row == savedInteger) {
        cell.accessoryType = UITableViewAccessoryCheckmark; 
    }

    return cell;
}