[iOS] Don't reconfigure cells if they are nil

When reconfiguring a "nil" cell, there is a DCHECK in the item.
This CL makes sure that we don't reconfigure if we the cell is nil.

Bug: 1013708
Change-Id: I1082a59a74fc50b0b0243a4ff9255606a3218c6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862675
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705912}

ios/chrome/browser/ui/reading_list/reading_list_egtest.mm

@@ -12,6 +12,7 @@
 #include "base/bind.h"
 #include "base/ios/ios_util.h"
 #include "base/mac/foundation_util.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #import "base/test/ios/wait_util.h"
@@ -274,6 +275,28 @@ void OpenReadingList() {
   [chrome_test_util::BrowserCommandDispatcherForMainBVC() showReadingList];
 }
 
+// Adds 20 read and 20 unread entries to the model, opens the reading list menu
+// and enter edit mode.
+void AddLotOfEntriesAndEnterEdit() {
+  ReadingListModel* model = GetReadingListModel();
+  for (NSInteger index = 0; index < 10; index++) {
+    GURL url_to_be_added =
+        GURL(kReadURL + std::string("/") + base::NumberToString(index));
+    model->AddEntry(url_to_be_added, std::string(kReadTitle),
+                    reading_list::ADDED_VIA_CURRENT_APP);
+    model->SetReadStatus(url_to_be_added, true);
+  }
+  for (NSInteger index = 0; index < 10; index++) {
+    GURL url_to_be_added =
+        GURL(kUnreadURL + std::string("/") + base::NumberToString(index));
+    model->AddEntry(url_to_be_added, std::string(kReadTitle),
+                    reading_list::ADDED_VIA_CURRENT_APP);
+  }
+  OpenReadingList();
+
+  TapToolbarButtonWithID(kReadingListToolbarEditButtonID);
+}
+
 // Adds a read and an unread entry to the model, opens the reading list menu and
 // enter edit mode.
 void AddEntriesAndEnterEdit() {
@@ -872,6 +895,21 @@ void AssertIsShowingDistillablePage(bool online, const GURL& distillable_url) {
   XCTAssertEqual((size_t)0, ModelReadSize(GetReadingListModel()));
 }
 
+// Marks all read entries as unread, when there is a lot of entries. This is to
+// prevent crbug.com/1013708 from regressing.
+- (void)testMarkAllUnreadLotOfEntry {
+  AddLotOfEntriesAndEnterEdit();
+
+  AssertToolbarMarkButtonText(IDS_IOS_READING_LIST_MARK_ALL_BUTTON);
+  TapToolbarButtonWithID(kReadingListToolbarMarkButtonID);
+
+  // Tap the action sheet.
+  TapContextMenuButtonWithA11yLabelID(
+      IDS_IOS_READING_LIST_MARK_ALL_UNREAD_ACTION);
+
+  AssertHeaderNotVisible(kReadHeader);
+}
+
 // Selects an unread entry and mark it as read.
 - (void)testMarkEntriesRead {
   AddEntriesAndEnterEdit();

ios/chrome/browser/ui/reading_list/reading_list_table_view_controller.mm

@@ -667,6 +667,9 @@ ReadingListSelectionState GetSelectionStateForSelectedCounts(
                     toSection:(SectionIdentifier)toSection {
   // Reconfigure cells, allowing the custom actions to be updated.
   for (NSIndexPath* indexPath in sortedIndexPaths) {
+    if (![self.tableView cellForRowAtIndexPath:indexPath])
+      continue;
+
     [[self.tableViewModel itemAtIndexPath:indexPath]
         configureCell:[self.tableView cellForRowAtIndexPath:indexPath]
            withStyler:self.styler];