[iOS] Stop loading Drive file picker items once disconnected

This CL returns early when loading Drive file picker items if the object
used to fetch items is unavailable, which means the mediator was
disconnected.

Change-Id: I3535f7553a292743408ec48c908154203df6616c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5928669
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1368263}

ios/chrome/browser/drive_file_picker/coordinator/drive_file_picker_mediator.mm

diff --git a/ios/chrome/browser/drive_file_picker/coordinator/drive_file_picker_mediator.mm b/ios/chrome/browser/drive_file_picker/coordinator/drive_file_picker_mediator.mm
index d04047d..99d3e99e 100644
--- a/ios/chrome/browser/drive_file_picker/coordinator/drive_file_picker_mediator.mm
+++ b/ios/chrome/browser/drive_file_picker/coordinator/drive_file_picker_mediator.mm
@@ -737,6 +737,11 @@
 - (void)loadItemsAppending:(BOOL)append
                    delayed:(BOOL)delayed
                   animated:(BOOL)animated {
+  if (!_driveList) {
+    // When disconnected, do nothing.
+    return;
+  }
+
   // If there is a timer programmed to fetch items, cancel it.
   _fetchTimer.Stop();
   // Cancel any pending fetch query.