[Downloads UI] Add null checks to prevent crash when selecting deleted items

If you move quickly enough, it's possible to select an item that was just
deleted then choose to delete it again. Add some null checks to prevent
a crash.

BUG=652176

Review-Url: https://codereview.chromium.org/2387003003
Cr-Commit-Position: refs/heads/master@{#422662}

chrome/android/java/src/org/chromium/chrome/browser/download/ui/DownloadManagerUi.java

@@ -429,6 +429,10 @@ public class DownloadManagerUi implements OnMenuItemClickListener {
                 mBackendProvider.getSelectionDelegate().getSelectedItems();
         final List<DownloadHistoryItemWrapper> itemsToDelete = getItemsForDeletion();
 
+        mBackendProvider.getSelectionDelegate().clearSelection();
+
+        if (itemsToDelete.isEmpty()) return;
+
         mHistoryAdapter.removeItemsFromAdapter(itemsToDelete);
 
         dismissUndoDeletionSnackbars();
@@ -445,8 +449,6 @@ public class DownloadManagerUi implements OnMenuItemClickListener {
         snackbar.setTemplateText(mActivity.getString(snackbarTemplateId));
 
         ((SnackbarManageable) mActivity).getSnackbarManager().showSnackbar(snackbar);
-
-        mBackendProvider.getSelectionDelegate().clearSelection();
     }
 
     private List<DownloadHistoryItemWrapper> getItemsForDeletion() {
@@ -457,7 +459,11 @@ public class DownloadManagerUi implements OnMenuItemClickListener {
 
         for (DownloadHistoryItemWrapper item : selectedItems) {
             if (!filePathsToRemove.contains(item.getFilePath())) {
-                itemsToRemove.addAll(mHistoryAdapter.getItemsForFilePath(item.getFilePath()));
+                List<DownloadHistoryItemWrapper> itemsForFilePath =
+                        mHistoryAdapter.getItemsForFilePath(item.getFilePath());
+                if (itemsForFilePath != null) {
+                    itemsToRemove.addAll(itemsForFilePath);
+                }
                 filePathsToRemove.add(item.getFilePath());
             }
         }