message: Made block nullable so can clear in stack
  Removes compiler warning
diff: @@ -30,7 +30,7 @@ NS_ASSUME_NONNULL_BEGIN
  @property (nonatomic, copy) UIColor *largeTitleColor;
  @property (nonatomic, copy) NSString *backTitle;
  @property (nonatomic, copy) NSString *backTestID;
  -@property (nonatomic, copy) void (^backImageDidLoadBlock)(void);
  +@property (nonatomic, copy) void (^ _Nullable backImageDidLoadBlock)(void);
  @end
  