Revert D6871289: [IGListKit][PR] Removed unnecessary nil check on transitionDelegate

Summary:
This reverts commit 7ccf5a286ab4ee63bf72273142857496ce62b6f4

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Differential Revision:
D6871289

Original commit changeset: 7ccf5a286ab4

fbshipit-source-id: e75cc7bd4a378e1092f4a01be3104fddb336759a

CHANGELOG.md

@@ -25,6 +25,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - Reordered position of intercepted selector in `IGListAdapterProxy`'s `isInterceptedSelector` method to reduce overall consumption of compare. [zhongwuzw](https://github.com/zhongwu) [(#1055)](https://github.com/Instagram/IGListKit/pull/1055)
 
+- Made IGListTransitionDelegate inherited from NSObject. [Igor Vasilenko](https://github.com/vasilenkoigor) [(#1075)](https://github.com/Instagram/IGListKit/pull/1075)
+
 ### Fixes
 
 - Duplicate objects for initial data source setup filtered out. [Mikhail Vashlyaev](https://github.com/yemodin) [(#993](https://github.com/Instagram/IGListKit/pull/993)

Source/Internal/IGListAdapter+UICollectionView.m

@@ -218,10 +218,13 @@
                    customizedInitialLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes
                                          atIndexPath:(NSIndexPath *)indexPath {
     IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];
-    return [sectionController.transitionDelegate listAdapter:self
-                           customizedInitialLayoutAttributes:attributes
-                                           sectionController:sectionController
-                                                     atIndex:indexPath.item];
+    if (sectionController.transitionDelegate) {
+        return [sectionController.transitionDelegate listAdapter:self
+                               customizedInitialLayoutAttributes:attributes
+                                               sectionController:sectionController
+                                                         atIndex:indexPath.item];
+    }
+    return attributes;
 }
 
 - (UICollectionViewLayoutAttributes *)collectionView:(UICollectionView *)collectionView
@@ -229,10 +232,13 @@
                      customizedFinalLayoutAttributes:(UICollectionViewLayoutAttributes *)attributes
                                          atIndexPath:(NSIndexPath *)indexPath {
     IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];
-    return [sectionController.transitionDelegate listAdapter:self
-                             customizedFinalLayoutAttributes:attributes
-                                           sectionController:sectionController
-                                                     atIndex:indexPath.item];
+    if (sectionController.transitionDelegate) {
+        return [sectionController.transitionDelegate listAdapter:self
+                                 customizedFinalLayoutAttributes:attributes
+                                               sectionController:sectionController
+                                                         atIndex:indexPath.item];
+    }
+    return attributes;
 }
 
 @end