Fix contact offer

We never assign interaction, but we can get it via the viewItem.

// FREEBIE

Signal/src/ViewControllers/ConversationView/Cells/OWSContactOffersCell.m

@@ -15,7 +15,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @interface OWSContactOffersCell ()
 
-@property (nonatomic, nullable) OWSContactOffersInteraction *interaction;
+@property (nonatomic, nullable, readonly) OWSContactOffersInteraction *interaction;
 
 @property (nonatomic) UILabel *titleLabel;
 @property (nonatomic) UIButton *addToContactsButton;
@@ -195,15 +195,19 @@ NS_ASSUME_NONNULL_BEGIN
     return result;
 }
 
-- (void)prepareForReuse
-{
-    [super prepareForReuse];
+#pragma mark - Events
 
-    self.interaction = nil;
+- (nullable OWSContactOffersInteraction *)interaction
+{
+    OWSAssert(self.viewItem);
+    OWSAssert(self.viewItem.interaction);
+    if (![self.viewItem.interaction isKindOfClass:[OWSContactOffersInteraction class]]) {
+        OWSFail(@"%@ expected OWSContactOffersInteraction but found: %@", self.logTag, self.viewItem.interaction);
+        return nil;
+    }
+    return (OWSContactOffersInteraction *)self.viewItem.interaction;
 }
 
-#pragma mark - Events
-
 - (void)addToContacts
 {
     OWSAssert(self.delegate);