Add production workaround for missing elements (#3158)

Source/ASCollectionView.mm

@@ -1740,14 +1740,22 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
   // Visible items
   for (NSIndexPath *indexPath in self.indexPathsForVisibleItems) {
     ASCollectionElement *element = [map elementForItemAtIndexPath:indexPath];
+    if (element != nil) {
       [result addObject:element];
+    } else {
+      ASDisplayNodeFailAssert(@"Couldn't find 'visible' item at index path %@ in map %@", indexPath, map);
+    }
   }
 
   // Visible supplementary elements
   for (NSString *kind in map.supplementaryElementKinds) {
     for (NSIndexPath *indexPath in [self asdk_indexPathsForVisibleSupplementaryElementsOfKind:kind]) {
       ASCollectionElement *element = [map supplementaryElementOfKind:kind atIndexPath:indexPath];
+      if (element != nil) {
         [result addObject:element];
+      } else {
+        ASDisplayNodeFailAssert(@"Couldn't find 'visible' supplementary element of kind %@ at index path %@ in map %@", kind, indexPath, map);
+      }
     }
   }
   return result;

Source/Private/ASElementMap.m

@@ -13,8 +13,9 @@
 #import <AsyncDisplayKit/ASMutableElementMap.h>
 #import <AsyncDisplayKit/ASSection.h>
 #import <AsyncDisplayKit/NSIndexSet+ASHelpers.h>
+#import <AsyncDisplayKit/ASObjectDescriptionHelpers.h>
 
-@interface ASElementMap ()
+@interface ASElementMap () <ASDescriptionProvider>
 
 @property (nonatomic, strong, readonly) NSArray<ASSection *> *sections;
 
@@ -168,6 +169,21 @@
   return [_elementToIndexPathMap countByEnumeratingWithState:state objects:buffer count:len];
 }
 
+#pragma mark - ASDescriptionProvider
+
+- (NSString *)description
+{
+  return ASObjectDescriptionMake(self, [self propertiesForDescription]);
+}
+
+- (NSMutableArray<NSDictionary *> *)propertiesForDescription
+{
+  NSMutableArray *result = [NSMutableArray array];
+  [result addObject:@{ @"items" : _sectionsOfItems }];
+  [result addObject:@{ @"supplementaryElements" : _supplementaryElements }];
+  return result;
+}
+
 #pragma mark - Internal
 
 /**