Respond to CR.

// FREEBIE

Signal/src/ViewControllers/OWSTableViewController.h

@@ -37,7 +37,7 @@ typedef NS_ENUM(NSInteger, OWSTableItemType) {
 };
 
 typedef void (^OWSTableActionBlock)();
-typedef UITableViewCell * (^OWSTableCustomCellBlock)();
+typedef UITableViewCell *_Nonnull (^OWSTableCustomCellBlock)();
 
 @interface OWSTableItem : NSObject
 
@@ -54,7 +54,7 @@ typedef UITableViewCell * (^OWSTableCustomCellBlock)();
 + (OWSTableItem *)itemWithCustomCellBlock:(OWSTableCustomCellBlock)customCellBlock
                               actionBlock:(nullable OWSTableActionBlock)actionBlock;
 
-- (UITableViewCell *)customCell;
+- (nullable UITableViewCell *)customCell;
 - (NSNumber *)customRowHeight;
 
 @end

Signal/src/ViewControllers/OWSTableViewController.m

@@ -141,7 +141,7 @@ NS_ASSUME_NONNULL_BEGIN
     return item;
 }
 
-- (UITableViewCell *)customCell
+- (nullable UITableViewCell *)customCell
 {
     if (_customCell) {
         return _customCell;