Can request CollectionViewModel itemAtIndexPath with nil

ios/chrome/browser/ui/collection_view/collection_view_model.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/collection_view/collection_view_model.h"
 
 #include "base/logging.h"
 #include "base/mac/scoped_nsobject.h"
 #import "ios/chrome/browser/ui/collection_view/cells/collection_view_item.h"
 
@@ skipping 129 matching lines @@
 
   CollectionViewItem* item = [self itemAtIndexPath:indexPath];
   NSUInteger indexInItemType =
       [self indexInItemTypeForItem:item inSectionItems:items];
   return indexInItemType;
 }
 
 #pragma mark Query items from index paths
 
 - (BOOL)hasItemAtIndexPath:(NSIndexPath*)indexPath {
+  if (!indexPath)
+    return NO;
+
   if (static_cast<NSUInteger>(indexPath.section) < [_sections count]) {
     SectionItems* items = [_sections objectAtIndex:indexPath.section];
     return static_cast<NSUInteger>(indexPath.item) < [items count];
   }
   return NO;
 }
 
 - (CollectionViewItem*)itemAtIndexPath:(NSIndexPath*)indexPath {
+  DCHECK(indexPath);
   DCHECK_LT(static_cast<NSUInteger>(indexPath.section), [_sections count]);
   SectionItems* items = [_sections objectAtIndex:indexPath.section];
 
   DCHECK_LT(static_cast<NSUInteger>(indexPath.item), [items count]);
   return [items objectAtIndex:indexPath.item];
 }
 
 - (CollectionViewItem*)headerForSection:(NSInteger)section {
   NSInteger sectionIdentifier = [self sectionIdentifierForSection:section];
   NSNumber* key = [NSNumber numberWithInteger:sectionIdentifier];
@@ skipping 148 matching lines @@
     }
     if (sectionItem.type == item.type) {
       indexInItemType++;
     }
   }
   DCHECK(found);
   return indexInItemType;
 }
 
 @end